I think many of the "comspec" AR makers like ArmaLite and Bushmaster who have been around 20 years or so just continued using the A2 FSB even though they saw increased sales over the years of flat top receivers and decreased sales of A2 receivers. To offset zeroing issues they sold rear sights/carry handles that also were A2 height (.04" shorter).

Today the vast majority of receivers are flat top/A4 and most rear sights are A4 height yet the comspec makers hold on to the A2 FSB probably not to confuse and anger their customers. They just hope no one will notice, and usually people let them get away with it.

Anyway, it's not right and no one buying a new rifle today should have to put up with this crap. And when selling a barrel with FSB, the vendor should clearly state A2 or F-marked.