Bookworm,
I'll take the bait...
The clue to the answer to your conundrum is the fact that the downgoing wing is operating at a higher AoA than the upgoing wing. With the ailerons deflected, we effectively have a different aerofoil section on each side, and while the roll rate remains constant, we also know that each wing is producing the same amount of lift. However, there's no reason why two different aerofoils operating at two different angles of attack should also produce the same amount of drag, just because they happen to be producing the same amount of lift - the amounts of drag from each wing are very likely to be different.
Pursuing this line of thought... the downgoing wing is a lower-camber aerofoil operating at a higher AoA, while the upgoing wing is a higher-camber aerofoil operating at a lower AoA... for the same value of L, would one expect the high-camber/low-AoA combination to always have a worse L/D ratio...?
I'm getting out of my depth, help!!
cbl