There is almost definately an element of luck to it, but half the reason "the sims are wrong" is because sometimes the spell priority you are meant to follow is excedingly difficult to perform as well as the sim does. Gear would also be a factor, their amount of crit and haste reduces the amount of luck they need to get those uptimes.
If we take that second log for example, they had 52.6% crit chance on frostbolt (So less IV cd), when their frostbolt should have about 49% crit chance, so it's slightly higher or "lucky". They also had 16 flurry casts from 95 frostbolts, which is actually a 17% chance of brainfreeze proc, where as it should have been 23% (15+6 from artifact wep, +2 from relic trait), and the reason this is relevant is because of the guaranteed frostbolt crit from flurry, as well as the ice lance to keep IV going.
They also have 2 frostbolt reduce the cd of IV relics in their weapon.
It may be that LW is an easier to execute priority list and therefore for most people will perform closer to what the sims suggest. Also, the glacial spike with LW has been proven to be effective, but it can be very RNG dependant on where you get your crits.
If you really want lots of information about what specs work, try
https://www.altered-time.com/forum/ it is basically the best place to find mage information.