I was shocked.
After switching to the prime, I found everything was 2/3 of a stop *more* exposed than with the same settings on the zoom. I double-checked all my settings to make sure I hadn't bumped anything while switching the lenses and found them to be identical.
Any idea why this would be happening? I've tested in various other situations and concluded the following:
a) It's not the specific copy of the prime or the zoom -- I've tested with other lenses at the same approximate focal length and found them to match my findings with original zoom and prime. All my zooms are letting in ~2/3 stop less light than the primes. I also have two copies of the 50mm prime and both perform identically.
b) It's not the studio lights. I've checked in an all natural situation.
I'm thinking it's either from the Active D Lighting on my D300 or it's because the primes simply let in more light. I'm open to other suggestions, though, since neither of these seems quite right.