shell oil charge me for more gas than i received, Internet

i went to the shell station located at the corner of san marcos blvd and rancho santa fe rd in san marcos, ca. on wednesday march 2, at around noon. with a one gallon four ounce can that had a little gas in the bottom, and i filled up to the fill line on the can, when i was done the receipt said $4.15 for the cost but the cost of a gallon was $3.799. it also said 1.093 gallons which would be impossible since the can only holds 132 oz. i know it's only a small amount you are ripping me off  but if it's so insignificant then why do you do it? why not let me have the extra gas. now you have ruined the economy with your greed for gas prices and in addition you have to rip us off a little too?