I don't think you will ever get exact numbers, way too many folks just don't play by the rules, and I do think the GC estimate is maybe CLOSE< but I don't think its 90% close/accurate either!
I know far too many guys that butcher there own deer, farmers that???
leaving that alone!
and well, guys just too lazy to call in or the likes.

I was at an auction not so long ago, and the estate being auctioned off, had about 5-7 mounted bucks's and a BIG box of antlers of smaller bucks.
and the auctioneer, even made the joke, a PA< hunter, look he had all his tags still on all his OLD PA lic, that we in a big box/
and the whole crowd started laughing, as sadly they know the reason?? as its unspoken words of the likes that is generations deep in many places in PA, its like a rights to passage in some area's to KILL more than one buck and or NOT use your tag .

But I do think this is being slowly changed here.
I personally wish there were check in required for deer like bears,

that would give better data I think on age's of deer and sizes, and at same time, get hands on info of hunters in area's of WHAT deer numbers are like.
when I hunted Ohio for yrs and yrs, it was nice, doing the check in's
there were TONS of places to do so, never seemed an issue to me, and I think local business's made more $$ off guys coming in to check in deer, even locals would stop in to look at pic's most took of kills.
I know ever time I checked a deer in there, I bought some things at the store during check in, drinks, ice, food ??

also got me to meet wardens there too, a plus to me, as they also got to meet and a few know me over the yrs, was a win win to me, but they went away from check in's so??