21% haste is a 26.58% increase in attack rounds. 22% haste is a 28.21% increase in attack rounds. - Or to put it simply a 1.63% increase in attack rounds.
This is incorrect. It's pretty much never in terms of addition/subtraction and most often in terms of multiplication/division.

Let's look at it this way.

Our average dmg is 1 dmg/attack.

We go for 100 attack rounds.

21% haste, you get 26.58% extra attacks for 126.58 attacks total.
22% haste you get 28.21% extra attacks for 128.21 attacks total.

Since our average dmg is 1/hit (1 is just easy to work w/ any number would give same results).

128.21 dmg vs 126.58 dmg.

You gain 1.63 damage, in comparison, but you don't a 1.63% increase in damage.

128.21/126.58 = 1.29% increase in damage.

In that particular scenario, you're giving it slightly more credit than what it's worth.