It's a problem because there's rounding. I'm aware that there's a table but that table is functionally irrelevant because the text specifies that half of a shugenja's spells for a level have to be from their favored element, and Wizards rounded up on the half. The table is just so player's don't have to think too hard about it or question which half gets put into the element when there's an odd number. You're right about the slot though, I've yet again failed to remember how extra slots for a high casting stat work.