There are three computer memories: 1, 2 and Auto. Memories 1 and 2 do not get reset unless the driver manually resets them. The Auto memory is reset every time you swtich off the engine. Obviously these memories will affect the MPG shown. You need to select which memory to display.
Now for the calculation itself, I believe that an average over a certain number of miles is computed. In my previous cars this was a rolling average over around 20 miles. I'm nit sure what it is in the XF but I wouldn't imagine it would be much different.
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