It goes back to the lower being shipped from the source to the FFL as a stripped unfinished receiver. The major reason the ATF changed the form was to eliminate putting stripped lowers into rifle or pistol because it hadn't been yet constructed.
Having the FFL determine what the lower would turn into is a big delegation of authority to someone who isn't on their payroll or who they trust to make a legal interpretation. The ATF only trusts themself to make that finding according to the regulatory whim of the moment - not the FFL.
IT MAKES NO DIFFERENCE WHAT WAS RECORDED ON THE FORM. THE FFL AND FORM ARE NOT A LEGAL DETERMINATION. ONLY THE ATF CAN MAKE THAT DETERMINATION. NOBODY ELSE. There is no case history of the ATF prosecuting people for what an FFL put on the form in error. There IS for being a felon or indulging in marijuana.
If there is a question they will go back to the maker, the state of finished assembly it was shipped in, and then how it was received by the owner. FFL's have nothing to do with it. They just check boxes on the form and call in NICS. Which should give you a better understanding of just how useless the whole thing is.
And THAT is the reason people buy 80% lowers and pay more to get them than a finished one with serial number. It's not illegal and sidesteps the entire circus of government intervention, all of which is a restriction on your 2A rights.