In general, anytime you hit a level cap for an expansion (50, 60, 70, etc.) there will be a large jump in damage potential. All of the patches between expansions increase the ilevel cap so you end up with a large range of ilevels at the level cap for that expansion. As you level up from 50 to 59 for example, the ilevel and gear level both gradually increase together but as soon as you hit level 60 you suddenly get access to 4 patches worth of level 60 ilevel upgrades at once allowing you to increase your ilevel a lot in a single go.
