Well, say for instance you pulled up to the pump to fill up with gas, and your odometer read 300.0 miles. However, in reality you traveled 300.08999 miles. In reality, you have traveled 469.92 feet farther than you're recording. Over the course of a couple of tanks, this is not noticeable. However, over many fill ups these odd amounts add up.
The only way to account for it is to use actual odometer readings instead of using the trip odometer. This way if you skip recording 500 feet on one fillup, they're picked up on the next tank. This variance can vary each time you fillup, so there's no set formula to account for it.