When your GPS watch and the course disagree
Use elapsed time and reliable course markers to cross-check your plan.
Why the numbers diverge
Tall buildings, curves, GPS error and the line you actually run can all affect the watch reading. Instant pace can swing even if your effort feels steady.
Use a backup
Print cumulative times for 5 km intervals. If you plan to use manual laps, practise the button sequence and decide whether to disable auto-lap before race day.
Example
If a marked 20 km checkpoint passes when the watch reads 20.4 km, compare elapsed time against the 20 km row of the pace band. Avoid resetting the watch mid-race just to make the distance match.