Breaking news… GPS isn’t 100% accurate!
I know as runners we often love the stats but it’s important to know that GPS isn’t 100% accurate. It depends on your watch/phone, the tech it uses, how often it takes a reading of your position, what satellite system it uses etc, as well as where you are running (trees and tall buildings don’t help)! GPS calculates your distances, and also therefore your pace/speed too.
Also some phones and activity trackers don’t use GPS to calculate distance at all they use your step count and an estimate of your stride length - which is even less accurate! A note on treadmills; your watch/phone can’t use GPS for treadmill runs as you aren’t moving so if it has a treadmill setting it will be using step rate and stride length as above. If a treadmill is regularly maintained and calibrated (that’s a big “if” in some gyms!) the treadmill reading is likely to be more accurate that your watch’s reading.
Race courses like London (and any certified by UK Athletics) have to be accurately checked and measured and that distance will be spot on for a specific “line” around the course. At London you’ll see a blue line which the elite athletes will follow which is exactly marathon distance long. As London is crowded and you won’t be running that shortest route around each corner you’ll likely run slightly over distance on the day.
If you want to learn more about the tech behind GPS you can watch a video I did about it here; https://youtu.be/uvDfiLTwunI