I guess the 76 just came out better with that fin combo in testing. I've used the 76 (as well as other sizes) with both 16/11 and 15/12 (and 15/11 to for that matter) and in th kind of soft conditions I've sailed them in all combos have worked well. SO I guess you need faster waves and/or more time to get into the subtle details that differ the two setups.
But I'm gonna ask Scotty to comment. He's the one that did the testing.