Unfortunately it was a common mistake (and probably still is) sometimes.
What happens in some cases, is the flare and fender do not match perfectly. Either you can get the top line to sit level, or you can get the lower lines to match the body. But not both!
After so many experiences with odd stuff on Broncos, it's hard to say if the fenders are at fault or the flares are wrong. Probably a bit of both. Maybe the '66 fenders were slightly different and that's what everybody molded their flares on? Dunno, but this uneven mismatch is common.
So the cure is to get it level (especially to the eye) and just let the trailing or leading edges fall where they may. That actually ends up being hardly noticeable compared to the uneven top line.
I would hate it myself, but I would not let it be a deal breaker necesssarily either. Depends on the condition of the rest of the Bronco.
And you said respected by most on this site. Is that from your own reading here? Or from what someone said?
Might be good to either let us know the name, or make sure you did your own searching. Plenty of good and well respected builders out there. Plenty of pikers too, so worth a second look.
Seems like someone familiar with EB's would have known how to trick the eye and get a flare on straighter.
He says cautiously, knowing it might be one of our customers!
Then again, in their favor, they might have been dealing with an already cut fender by a PO and simply going with the same cut rather than fixing or replacing the fender.
Paul