[IYAS9YAS]

there is some sort of caveat on a small label on gas pumps which may speak to this apparent discrepancy, but i cannot recall the wording oft hand.

Fill in the morning. There are net/gross volumes for fuel. Taxes are generally calculated on the gross gallons (which is what the fuel would be measured at at 60 degrees F, and sea level), then the net is figured on actuals. The difference can be up to 100 gallons on a 10,000 gallon load. But it averages out over time, due to seasonal fluctuations. More net in winter than gross, more gross in summer than net.

Used to work in the field, and it's been more than 10 years since, but IIRC, that's how it worked.

[thackney]

Fill in the morning.

Because the underground tank goes through temperature swings day to night?

[45semi]

On a TLS-350 Veeder -350 tank monitor system, the expansion rate is .070 for gas and .068 for diesel.
Now, there IS more butane in 'winter' fuel. So if you store gas, it's best to buy it in the winter.

http://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/summer-fuel1.htm