Fuel gauge... HELP!

VancouverZ34

I left my car in an underground parking lot for little over 2 months.
During that period, I never filled up my gas tank (since I filled it up to the full when I started this storage)
I regularily ran the engine for about 10 minutes at least once a week.
Also, I took it out once and put about 40 miles on it on a highway.
Yesterday, I filled up the gas again since I noticed I only had half of gas left + I will be storing my Z for another month.
The problem I noticed was that the fuel gauge didn't go all the way up even after making sure gas was full the the tank by clicking gas pump few more times to see gas over flowing (I know, but I had to do this to make sure my car gets full tank of gas)
Anyone has an idea on what is causing this wrong reading on fuel level?
and how to fix it?