Odd Issue Pertaining to Gas

For a second, just a second... I thought maybe there was a possibility that my e46 was for once in line. I thought wrong, so today I went and got my car washed (which sucked because my trunk emblem sticker ripped in half... wow) and decided to run by the gas station. My levels were showing about a third of a tank on the dash. So, I put 20 dollars on my pump, etc. As the gas is pumping it stops at $12, I knew this was wrong so I just continued to fill until I got my $20 worth. To my surprise, my dash was showing that I was about 3 quarters of a tank.

What's the problem here? I'm sure it's some sensor associated with the fuel tank. Anyone ever have this issue? I've never seen anything like this on the forum before so I must say I'm shocked.