Special attacks, yellow damage, are on a two roll system. Auto-attack are the only things on a single roll system. In other words instant attacks have to hit to crit.

I saw the same thing with Crusader Strike leveling my Ret paladin back in TBC. An attack larger than normal but smaller than a crit which turned out to be A Blocked Crit.

The reason you can fail to crit at 100% crit chance is that, for whatever reason, some mobs reduce the chance that you'll crit them by more than you might expect.


Or you miss.