MacOS MPH Accuracy

I have used Fulgaz for over a year and over the past 5 months have noticed a 1.5-2mph difference in the actual speed I am riding and the average mph that Fulgaz calculates. For example, on one of my recent rides my current mph was always 18.5 or above, but the average mph and distance was at 17. It is frustrating that the information is not accurate.