What they said *points up*
The quirk is that NWN doesn't reveal what class is casting a spell when that spell is cast. It's largely irrelevant when a spell only belongs to one particular class, but when it's something like GMW, there's a difference in spell levels between Wizard/Sorcerer and Cleric. In such cases, Eschew always uses the highest spell level across all classes to determine whether or not it applies to any given spell.