I have read a lot and talked to people that have said their Garmin or Nike+ have said they've gone farther in races than what the track should have been. I think I finally know why that is. I was reading the newest addition of Runner's World and I found this:
" USATF-certified race courses are measured along the "shortest possible route that a runner could take and not be disqualified". This ensures that an athlete who cuts every tangent perfectly, who doesn't take one unnecessary step, still will have run at least 26.2 miles... One researcher estimates that marathoners can wind up running .6 miles longer than 26.2 miles."