The first two charts are the most used skills. You can see what the 6 skills are in the table at the bottom. It's only to illustrate the differences between the classes.
Factors contributed to changes and status quos, one example in parantheses.
Skills buffs (DH Perfectionist)
Skills nerfs (Wiz Storm Chaser rune)
Inaction from Blizzard (Monk)
Better gear available due to deflation of mid-high items and increased drop rate. (Witch Doctor increase skill distribution)
Monster Power system (Barb Thrive on Chaos)