Miata Fuel Gauge Miraculously Healed?

I've got a '96 Miata that for a few years now has had an issue with the fuel gauge. Specifically, it wouldn't register completely full. If fuel was coming out of the filler neck it indicated about 7/8, i.e., halfway between 3/4 and full. When it was on empty it would only take about 10.5 gallons (indicating about 1.5 left in the tank). I saw a thread on Miata.net about it when it first started doing this and didn't feel like doing whatever the fix was (something about putting a resistor in parallel with the sender or something).

Anyhow, the car was recently in the paint shop to fix some peeling clearcoat on the hood and finish panel. When I got the car back, the car indicated about 1/8 less than what I'd left it with, even though it had only been driven a mile or so. I figured they'd left it running and/or done a lot of cold starts when moving it around the paint shop.

I went to fill it up today for the first time in six weeks or so, and--lo, and behold--the gauge went all the way to F. So, what is the source/mode of this miraculous healing? I know that they had the battery disconnected because my radio had lost its mind.

maybe they disconnected the ground that the gage used and when they rehooked it up, made a better connection?

Nothing beats the time I did some welding on my first spider.. and accidently welded the fuel gage wires to the left turnsignal.. everytime I made a left, the gage would flip from ful to empty to full to empty......etc etc etc

That makes sense. They repainted the rear finish panel and the rear bumper cover. That very well may have involved undoing and redoing the ground for the fuel sender.

Yep bad grounds sure do suck.

If I ever build a car it's gonna have all the switches on the positive wires, and dedicated ground wires going back to the battery for everything...and not just because it would be made of mostly nonconductive materials.

I know on a fiat Spider 2000, the left taillight is the main grounding point for everything at the rear of the car. If it is loose or corroded, all sorts of strange things can happen.. including no start because the fuel pump was not running