Ammeter "woggle"

I was having a long standing problem on my 172N
with the ammeter "woggling", that is, rapidly
wiggling back and forth in level flight, through about
1/2 of the total scale. This was going on for a year
or more. On the last annual, the AP had me disassemble
the solenoids and brush the leads. There was, in fact,
considerable corrosion on the leads.

That fixed the problem. Now I wonder if someone has a
good explanation for why it was doing this. I'm not
an analog electronics genius, I figure the next effect
of the corrosion was to add a resistor in series to the
system. Why would this result in the strange ammeter
behavior ?