fuel gauge and miles remaining

[ohhh my]

About a month ago, my fuel gauge would only read up to 3/4 tank but the tank was surely full due to a little overfillage at the pump and following pumps would equate to approximately how many miles I had used on on that tank.

I found a little trick to get the needle to go all the way to full. The trick is to have the ignition in the on position (with engine off of course) while pumping gas and this led to a full tank of gas reading.

This last tank, I did ALL city driving and when I went to pump gas today, it read full but the miles remaining was around 210, which is what I got on my all city driving on my previous tank.

In the past, it would usually read 300+ does the computer base the milage remaining based upon your previous fill up? I'm assuming no? I'm assuming the computer just has a read out of milage remaining based upon the amount of fuel in tank. Am I correct? Any reason why I pumped 15 gallons today(ran it really low due to a busy schedule) but the milage remaining is only at 210?