Cooldown Modification math?

[Lerrasien]

So, I recently started playing and decided yesterday to start trying Ice Staff out. I really liked it other than how slow it is. So I decided to grab a book and throw cast timer mod food and passives. But while my math shows one thing, the game shows something completely different. And I'm not really sure where they're getting their math.

Then I looked at it with JUST the book on, which was supposed to give me 24% cast time mod and it only gave me 19%... Now... I've played A LOT of games. And I figured at first, that maybe it was just like Path of Exile, where you got the primary boost and then the rest built off of that. But if even the base boost isn't correct, then there's no way that's how their logic is... So I'm left completely baffled as to how they come up with the cast time modification boost math. Every other modification has been straight forward until this and it leaves me wondering if it was on purpose or if the person doing it was just drunk and never got called out afterward.