Our courses are certified to length by USATF standards. GPS tracking increases variance and a most runners will not run true/perfect tangents to maintain the most accurate reading either. When moving around other runners, changing lanes for water stops, gel, ports-johns, additional mileage is being accrued over the measured course. It’s not unusual to see variances in GPS distances from .2 to .5 and sometimes more, depending on the length of the race and how “on course” one stayed.