Just from your last statement I can guess your main class.
10->15 in particular with fortified makes a hude difference if you play with normal classes with rump-up AoE.
Your eyebeam/bladeflurry is the only thing hitting low HP mobs till they instantly die in a 10+, while for a real huge pop-up with wings/unholy-wounds/breath of sindragosa the meta-burst specs gets the dump in overall damage done when the trash is living for more than 1-2 seconds.
My opinion to this topic is, it mostly depends on your tank and pullsize to specific times.
Big pulls every 1.5min is unholy dk's wet dream scenario. Outlaw rogues with BLOOD and on-use 1.5min trinket cant even peak for half of unholys end DPS in such pulls.
Big pulls every 2min and frost-dk/rets gona end up with insane dps in overall, since they cleave with their 2min cooldowns pretty hard.
Small pulls chained and the only thing you will see is outlaw/dh/bm-hunter in top numbers.
If your PUG tank is just a random ignorant player, you will most likely just see dh/outlaw/bm on top, since those dont have big cooldowns but can pad pretty easy on low keys with the instant burst.
Thats M+ for you. Stupid players get better results with easy meta-specs in the group. Who would have thought?