This may explain why you get discrepancies in exposure:
The FA has a closed-loop exposure system. This means that while displaying the exposure any time the meter is on, the FA measures the actual exposure after the lens stops down to its taking aperture, just an instant before the actual exposure. This way the FA corrects for any errors in the lens' diaphragm mechanism, so I get exposures more consistent from lens to lens than on newer cameras. Some lenses have diaphragms which are calibrated so that they may over or under expose a little, but on an FA, they all expose perfectly.
http://www.kenrockwell.com/nikon/fa.htm

As you are using manual AI/-S lenses, there are tiny differences when the diaphragm closes. The FA is the only Nikon that corrects that difference with manual lenses. Others, like the F4 have a open-loop exposure system.