I've experienced that variation in gas port sizes first hand. The .073 was on a .625" ELW barrel and the .076 was on a .750" govt profile barrel. The only explanation I could find is an old thread on either LF or M4C that alluded to the theory that they went with the larger port on the larger diameter barrel to offset the gas traveling that extra .0625" through the gas port. That was a theory and not from BCM.
It makes sense, but at the same time seems like it wouldn't or shouldn't matter unless it was on the ragged edge of reliability, which anything with a .07 measurement on an 11.5" barrel is certainly not on the edge of reliability.
@Totem Polar every BCM upper I have owned has been great, even if they are a little generous with the gas port. It's not that bad, and nothing like the OG MK18 with the .082" gas port that chewed through gas rings every 2500 rounds. If you find one I think you'd be well served by it.
With that said, my 6933 absolutely dunked on the BCM 11.5" uppers I had. It was softer shooting while still maintaining reliability across many different ammo types, and consistently held tighter groups at 100.