In true AD&D the highest level spell a character could cast was determined by his/her intelligence.
e.g. 18 intelligence required to cast level 9 spells.
I'm pretty sure that this wasnt implemented in Baldurs Gate, though the manual writers were VERY lazy and this is yet another mistake they made.
Nothing more than that to add I think. I could mumble on about learning percentiles for spells, but that wasnt the question
