Camera meter adjustment.

I know that this may seem like odd question or a simple one but curious minds want to know. How does the meter in a camera get out of whack?
I have an old Nikkormat FTN that is in really good condition as far as the cosmetics and the majority of the mechanicals. However the light meter is a full stop off. IE when I set it to 400 and compare it to my other 2 cameras which the meter is on the nose I have to set it at 200. Yet I have a beater FTN that is ugly as my dogs butt, and it is right on the money. The pictures that I get back are very well exposed so I am assuming that the shutter is still pretty accurate. I know that I could get it repaired but I paid almost nothing for the body so it would be a waste to spend any cash on it. I can just adjust the film speed to match a known meter. It just doesn't make sense to me as to how a circuit gets out of adjustment.
And before anyone gets into the battery issue, I have the proper voltage battery in it. I use zinc cells so the voltage isn't likely the culprit.

A lot of photographers have 'stickies' on their cameras to compensate for meter irregularities. Set it at 200, check it for linearity (different light levels) and if it tracks, put a piece of tape on the camera indicating proper setting for the film you use most often.

I have a number of cameras and the in-camera meters only agree on a scant few. Different mfrs. use different methods to cal their meters and electronic components will change with age. Potentiometers, variable resistors used to calibrate circuits, will develop contact resistance and change the overall value.

Not a problem.

Fred

As to the how & why.
The components actually do degrade with age. CDS cells any how. And on the nikkormats there's a resistor band surrounding the lens mount that basically tarnishes over time. Tarnish=increased resistance or intermittent contact/erratic jumpy meter needle..They can be cleaned but there is timing via a gear drive that has to be correct on reassembly or you have even more problems than you started with.