Often, I see people complain about their class, stating that, because their class or spec doesn't appear in the top 100 of a certain fight, their spec is bad. this is a fake fallacy. people are forgetting that WoW is an RPG, and RPGs are notorious for one thing: RNG.
i'm going to use a general example using bland terms. suppose you are someone of class B, and your friend is of class A. there are 100,000 people who play each class. the numbers might be slightly off here and there, but not by so much that correct numbers would invalidate it.
Class A is a critproc sorceror: he relies on crits that have a chance to proc a spell that does much damage. the spell he fires does 100 damage every 3 seconds, has a 30% chance to crit for double damage, and those crits have a 50% chance to automatically cast an instant stronger spell that does double the damage of the original spell, and has 30% chance to crit as well, which has another 50% chance to proc the spell again, but those spells cannot proc it again. so for every 1000 of the first spell he casts, he does 700 normal hits for 100 each and 300 crits for 200 each. of those 300 crits, 100 proc a spell hit and another 50 proc a spell crit, and of those 17 proc the spell hit again, and another 8 procs of procs crit. so he does 70K from hit, 60K from crit, 20K from crit.proc.hit, 20K from crit.proc.crit, 3.4K from crit.proc.crit.proc.hit and 3.2K from crit.proc.crit.proc.crit, for a total of 176600 damage.
class B is a channeldot cleric: he casts 1 damage over time spell that does 77 damage every 3 seconds for 18 seconds and then does a channel that does 69 damage every 3 seconds over 15 seconds and has a 5% chance to proc an extra tick. he also has a 30% crit chance. he casts the dot and then does a 15 second channel and repeats this every 18 seconds. so over 1002 ticks of his DoT ( about same time period as the sorceror above), he does 100.3K damage from the DoT and can cast 167 channels, doing 835 ticks for 74900, with 42 extra ticks for a total of 3767 damage from the procs and a total damage of 178967 damage. he does steady damage.
so in the average scenario, B does slightly more damage than A, about 1%. however, you can clearly see that A is much more reliant on RNG. If B had a 10% increased proc rate, he would have 108K from the dot, 80661 from the channel and 4057 from the extra ticks, for a total of 192718. however, if A experienced a 10% higher crit rate, his spell distribution would shift from 600 normal hits for 100 each, 400 crit for 200, 120 proc hits for 200, 80 proc crits for 400, 24 proc of proc hits for 200 and 16 proc of proc crits for 400. the resulting numbers are 60K, 80k, 24K, 32k, 4.8K and 6.4K, for a total of 207,200. so by giving both a 10% extra crit rate, the gap between them has switched from 1% in favor of B, to 7% in the favor of A.
you might think: "But Nzall, we're comparing equal gear and fight length, how could the amounts of crits be different?" that's where RNG comes in. because you never cast spells for 3000 seconds in a fight. you often cast only 1/10th of that time, which means only 100 spells and therefore much more susceptible to RNG. another factor is that your crits aren't evenly distributed. sometimes you get lucky, and 40 of your 100 spells crit. other times, you get unlucky and only 25 of your 100 spells crit. that's the devil of RNG.
if we assume that every distribution of crits from 20% to 40% has 5% chance of occuring across all players, that means that 5,000 players of class B will do 1% higher damage than their comrades from A, but also 5,000 players of class A will have 7% higher damage than their friends from B. but on WoL, you'll only see the 100 best players overall, so you're only going to see 100 players of class A, while their average DPS with a gear-related critrate is lower than that of B. remember, both players had the same gear, the same stats and the same practical crit rate.
the same happens in WoW. for example, back in WotLK fire mages appeared on those logs as dominant, because their range of DPS encompassed a much larger range simply due to RNG. those players weren't just skilled or geared, they also had the luck to get that nice hot streak chain of pyroblast crits. meanwhile, elemental shamans had a strict rotation that wasn't as affected by crit or RNG, and had a much smaller DPS range. so if you had a mage and a shaman of equal gear and skill (and assume that their average DPS was balanced exactly the same), and a lot of luck for both, the mage would always be on top because he simply got a larger benefit from being lucky.
you see that in current logs as well. take the theoretical crit rates and proc rates the top 10 players have raid buffed, and compare it to the actual observed proc and crit rates in their logs, and you will notice that most of them were rather lucky as well. they had more crits, or a spell with a 10% proc rate proccing 15% of the time, and their trinkets always appear to line up with damage boost phases and cooldowns active. in addition, you will nearly always see the same classes leading, because they get a much bigger bonus from that extra luck.
luck is a factor in this game, and you shouldn't discount it. blizzard balances around average DPS, because if they were to balance around maximum DPS with maximum luck, you would either have every spec play exactly the same with exactly the same stats, or the average DPS would not be the same and thus instead of disparities at the top 5%, you would have disparities on the middle 50%, where the majority of the players are.
so next time that you complain that your class is not competitive because it isn't represented in the top 100, take a step back and consider that not everyone is equally affected by luck. Blizzard can certainly balance everyone's average DPS around a certain point, but they can't balance both average and maximum DPS at the same time without massive homogenization.