Let me step in here.

You can see a difference between tachs, I lost RPM when I got a new tach when my old tach from the same manufacturer met it's demise. RPM should be a ball park and not an absolute.

Pros make a HUGE difference. An APC is going to retain it's shape at higher RPM and air load than some other thinner props and that will also give you a lower reading for the same diameter and pitch from manufacturer to manufacturer.

Higher nitro content will give you a higher top RPM by a few hundred RPM depending on nitro content. Higher nitro will also give you a broader sweet spot on the needle at the top end and bottom end.

As for the FZ vs. FZs, the FZs made slightly more power but functionally they are the same.

Your RPM readings in your first post are in the ball park and certainly valid for 20% nitro. For comparison I would swing a 15x10 APC at 8800 RPM on 20/20 on the nose of a 1.20FZ.