Today I discovered something different about my Fieros. The deck grills on my 88GT are off being repainted and I want to drive the car Saturday. So I switched the deck grills from my 87 GT and found that the "thumb" screws that hold the grill down are in the second/inner mounting hole in the grill for the 88 rather than the outer mounting hole for the 87.. I always wondered why there were two different holes in the grills. Just a bit of trivia.
I actually ran across this oddity some time ago when I was doing a fastback GT body work swap on an '88 car using pre-'88 fastback GT parts.
It's likely as above - since the strut hats had to be relocated and different brackets made to hold the decklid vents, GM probably figured it was just easier to make more holes to mount, rather than try and completely redesign the way the decklid vents attached to the strut hat brackets.