So the fps counter is a theoretical number showing how many frames of
animation the system can display as a maximum.

Wrong. Very wrong. The FPS counter is the *actual* number of frames
being displayed in a given second. Depending on the implementation of the
counter, it may be averaged over the last few seconds, which reduces the
instantaneous accuracy. But it is still directly measuring the video card
performance.


Wrong, very wrong.
At least in the scenario of MSFS the fps counter bears no relation to what
your videocard puts out.
If it did you'd see it reflect stutters, but it doesn't