Spellbook ability bugs

Alright, I'm trying to make a skill for a hero that is based off the "spellbook" ability. Every time he gains a level, he learns another spell.

Here's the thing though: Whenever the hero spends another point the the "spellbook" skill, all the icons inside the book randomly appear, disappear and change places.

I'd be really happy to know WHAT causes this, because this is an important part of the map I'm making.

Thanks.

check that the minimum and maximum number of spells are the same. having a range makes the abilities given random.