Weird things going on with spell cost

[AbsoluteZero]

With 10 death books + specialist + conjuror summon ghouls costs 52. This is 59% the original cost ( 88 )

This doesn't make sense any way I work this out.

the total cost reduction should be 10 + 12 + 25 = 47%

Which means the cost should be 46.

So what's going on here? Even if I break things up and subtract the percentages one by one I still don't get 52.

[AbsoluteZero]

Ah wait, i guess the percentages are taken off one after another. I just did the math on that and got 52. Weird, why do it that way?

[Anskiy]

Seravy thought additive discounts were too powerful.

[martinl_00]

Seravy thought additive discounts were too powerful.

This also allows multiple discounts that sum to over 100% to exist in the game without breaking it.

Five combined 25% discounts are a 76% total discount the current multiplicative way, but the additive way they sum to -25% which is Bad.