What most people expect is:
<hasted cast time> = <normal cast time> * (1 – <haste %>)
which would work with 33%. However, this is not how haste works.

If we think about it, there are two ways to view the meaning of the word “haste”. The first way is to interpret it something along the lines of “take less time to do a set task”, and the second way is more akin to “do more in a set amount of time”. Generally speaking, when we think of haste, we’re likely to think about the first interpretation. For example, “Max hastily made his bed” most likely means that although it normally takes Max 3 minutes to make his bed, in this case he managed to make it in 2 minutes because he rushed. However, WoW uses the second interpretation instead of the first. An example of this interpretation would be “If I work with great haste, I should be able to paint 3 walls before dinner instead of 2.”

So, you’re probably thinking “Well, what’s the difference? Taking less time to do something is basically the same thing as doing more things in a certain amount of time!” It turns out, there’s a significant difference, mathematically speaking. Let’s say Al and Bob are cutting carrots, and it takes both of them about 1 minute (60 seconds) to cut 10 carrots. After a while, Al and Bob get bored, and so Al decides to cut his carrots in 50% less time while Bob decides to cut 50% more carrots each minute. If there was no difference between the two, then Al and Bob should still be cutting carrots at the same rate, right? Well, Al begins cutting 10 carrots in 60 – 50% = 60 – 30 = 30 seconds, and Bob begins cutting 10 + 50% = 10 + 5 = 15 carrots in 60 seconds. After 1 minute, Al ends up having cut 20 carrots while Bob has only managed 15.

However, if you try thinking about it with the other interpretation—that it allows you to cast more spells in a set amount of time—it begins to make much more sense. The way I conceptualize it is that haste increases your casting rate. So, if you have 100% haste, then you can cast twice as many spells in the same amount of time it would take you to cast a single spell with no haste. If we use a 3-second spell as an example, then 1 spell per 3 seconds + 100% = 1 spell per 3 seconds + 1 spell per 3 seconds = 2 spells per 3 seconds. This also means that if you want to reduce a 1.5 second spell cast or the 1.5 second global cooldown to 1 second, you would need 50% haste. If you conceptualize it with the formula just above this, you can math it out:

1 spell per 1.5 seconds + ? = 1 spell per 1 second
1 spell per 1.5 seconds + ? = 1.5 spells per 1.5 seconds
? = 1.5 spells per 1.5 seconds – 1 spell per 1.5 seconds
? = 0.5 spells per 1.5 seconds

0.5 spells per 1.5 seconds is what % of 1 spell per 1.5 seconds?
(0.5 spells per 1.5 seconds) / (1 spell per 1.5 seconds) = 1/2 = 0.5 = 50%

So, you need 50% haste to reduce a 1.5 second spell cast or global cooldown to a 1 second cast or global cooldown.