That's the miles left until empty tank? If so it's driven by how much fuel is in the tank. When you restart the car it prob reads the fuel level and calculates range. So a 10 mile difference is just a half gallon which is prob well within the tank fuel level measuring error

 

It's not just based on fuel in the tank. The computer calculates the range based on current (if driving) or recent fuel consumption. If you hot-rod around it calculates a much shorter range than during highway cruising.

 

Correct. It's estimated fuel consumption based on recent driving x fuel remaining in the tank

But there is no new driving history between turning off the engine and turning it back on again. So I would imagine it's just the volume reading in the tank fluctuating.

 

Let me try again. The fuel sensor in the tank is a good deal more precise than suggested. The range estimate given is tied to the average mpg readings given. When driving these are largely (but not completely) computed on usage during the current "trip," which is to say since the ignition was turned on or in some cases the trip computer reset. After a shutdown period following a short "trip" the range will be computed based on a longer history. The OP mentions a 6 mile drive of 20 minutes, indicating some stop-and-go. During and immediately after this trip the range will be computed on the poorer gas mileage happening during this drive. Later when the ignition is again turned on the range is calculated from a larger and more representative sample size and is therefore more accurate.