Your June 15th fillup is marked as a partial. When you mark a fillup as a partial there is no MPG calculation for that fillup, or the next (as you have no idea exactly how much fuel was used to drive the miles recorded on the odometer). This data is used for your overall mileage calculations though. There is no error.
Actually, there is an error, of sorts. Although there is no way to calculate the MPG on a partial fillup, you can calculate the total MPG on the partial and the next fillup together. Simply divide the sum of the miles for the partial and next full fillup by the sum of the gallons for the partial and next full fillup. This is an apparent flaw in the fuelly algorithm, but it may be intentional since they may not want to imply that the MPG for the next full fillup is for that fillup alone. Still, I think this could easily be accounted for with an asterisk and a footnote.