Why is my Apple Watch showing less mileage than I ran?

I’ve owned an Apple Watch SE for over 2 years. My main sport is running. Recently, during a half marathon, I realized the watch was showing my paces to be slower than what I know I was going, and by the end of the race I had only registered about 12.85 miles while my friends had 13.2 .. I reviewed the map and noticed there are little spaces where the run kinda cuts so this translates into the watch showing less mileage than what I actually run .. and I started checking previous runs and it has happened on a lot of them!!!