Unusual (But Good!!) Data Lately

Lately i have to wonder if the on board computer could possibly need some calibration. or maybe the bladder in my car is shrinking as the weather gets warmer...something is going on and i am puzzled.

here is the situation.

last 4 fillups

computer gas pump
51.2 54.69
57.0 59.19
54.4 55.28
55.2 59.31

i have never had a situation where one reading or the other was over more than 3 times in a row. usually they take turns being higher because of inconsistent fill levels. but its become so lopsided lately that now my pencil and paper mileage is higher than the computer mileage figures. anyone else see this? most here seem to be lower on the pump figures if the two dont match over the long run when computing mileage.

only thing i have done differently is i did get gas at a different station than my normal one (Arco on Gravelly Lake Blvd in Lakewood, WA) twice during this period. i realize that not trying to top off can result in a large swing between tanks because consistent fill volumes can conceivably have more variance, but i have been doing this nearly a year and have been pretty consistent so far. but if this keeps up, i will make the Gravelly Lake station my new default station since it seems to be more generous than mine