This is not the sequence of events for all aperture priority bodies. Imagine the shutter lag this would add to camera that use a mirror relay meter! Also, the whole point of Nikon's AI-S 'upgrade' with respect to aperture linearity would be lost. What you are experiencing is due to the FG having some elements of the FA's 'closed loop' exposure system- if it detects that the lens isn't AI-S, it tries to correct the exposure.
Originally Posted by Les Sarile