GPS devices are not sensitive enough to measure a course as accurately as the techniques used to lay out and certify the course.
NYCRUNS courses are measured using a device called a Jones Counter to determine the shortest possible route on a given roadway. The measuring process also includes a “Short Course Prevention Factor,” that adds a few feet for every mile. So, the course as measured should always be a little long. Plus, runners are not capable of running the shortest possible route perfectly. For example, in order for you to run a perfect 10K, you would have to run a straight line. So, the more turns a course has, the more distance you are going to add, (You’re only human!).
As for your GPS device, even though the devices are much improved, in the best of circumstances, they are just not capable of being that precise. There are other factors that can impact GPS as well – tall buildings, the weather, etc.
Finally, there are choices the race director might make that could lengthen a course, (never shorter!). For instance, if there is a last-minute construction project on one side of the street, you might avoid it and add a bit of distance in the process. However, we’re talking dozens of feet at most, not tenths of a mile.
That said, if you use your GPS in a race, there is a way to get improved performance. Instead of relying on your device to note splits via auto-lap, you can turn auto-lap off and do this manually in line with each mile marker. The locations of mile markers are documented as part of the official USATF-Certified course map as well (which are available on each race website page), so you can feel confident that if you press the split button at the first mile marker, you ran one mile in whatever time your device says. Same thing when you reach mile 2 and so on.
If you want to know more about the course you ran or you plan to run, you can find most certified courses in the USATF database. You’ll see in some cases that your favorite courses are redone annually reflecting change(s). However, certifications are good for ten years.
So, there you have it. Don’t throw out your GPS but respect its limitations.