I agree that it is rather weird that they made it that way with attack speed.

Regarding speeds you have 2 types which together become your total attack speed:
-The time it takes to do the attack animation
-The waiting time after attacking

So the biggest reason aria of speed differs in power is because it makes the animation faster. Why they chose to have two different speeds is beyond me.

In a way it makes "sense" though, let's take a mage. The mage charges up a fireball! The mage is now tired and must wait to charge another one. One speed affects how fast he charges up the fireball and the other affects how long he must rest before starting to cast another :P