Curious circumstance with fuel gauge?

[mikejudes]

Ok, it breaks down like this, this morning I had what was showing as 46 miles left in my tank. I still had to drive about the same to get to and from work so I literally just got a splash of $5.00 of 93octane which gave me an additional 1.2 gallons in the tank/ (Going to BJ's this evening to fill up with .70$ off a gallon so I wanted to come in limping on empty).
Anyway, when I got back in the truck after I put in another 1.2 gallons, my fuel gauge actually showed that I went down to 38 miles left - like it didn't pick up any additional gas and 8 miles less miles to drive.
Riddle me this Batman?

I know there are bigger fish to fry in this world we live in. I was just scratchin' my head on this one....... Thoughts?