Troll Lord Games

Wizards and Illusionists have identical XP progressions until the 11th level. Then the Illusionist requires only150K XP / lvl while the Wizard needs 250K. Why the difference? Are the Wizard's spells at level 12 and above more powerful than the Illusionist's?

The only person who can give an official reason for the XP charts is Davis, and it is unlikely he will come here and respond, as he is very busy and probably doesn't want to rationalize it as it is not something that is, necessarily, formulaic. Also, the Trolls aren't exactly keen on breaking this stuff down... I would know.
The other "qualified" person would be me, and there are two ways to look at it --

Illusionists, on the whole, with some exceptions now and then, are considerably "weaker" than wizards - especially with a Castle Keeper that is extremely strict in their use of illusionist spells. This means the XP charts can be, and often are, totally wrong and the reduced rate at higher levels is a type of reward for suffering the high cost at low levels.
Or, and this is the most important one --

Classes do not wager against each other. Their charts are derived solely on their abilities, so the advancement scheme has no correlation to that of the wizard, at all. This means the abilities obtained do not necessitate an increased XP chart because they are not "valued" at more than what they are - so, you can draw the conclusion this makes them "weaker" than the wizard, but it is not necessarily so.

wizards are more powerful at high levels