The important thing to know is that your overall average isn't simply an average of all your fuel-up MPGs. The calculation uses your total distance driven and your total fuel filled. So yes, a partial fuel-up doesn't have an individual MPG calculation, but the fuel and distance from that fill is included in your overall average.
So it can work out sometimes that your overall average is slightly higher than any one fuel-up average. This slightly odd effect tends to go away once you have a dozen fuel-ups or so.