Strava and Apple Watch - GPS Inaccuracies

Hi all. Quick question here I’m sure someone can answer.

I run regularly, and when I compete in my local 5km race (which is exactly 5km) for some reason using Strava on my Watch Series 2, always results in a reading of 4.9km. Any idea why this is? I have my phone in my pocket as I run, and someone has suggested that it is this which is causing the problem - Strava is using the GPS on the phone which is not as accurate, but I struggle to believe that given I begin the process on the Watch? Might it just be me not starting/it not starting as soon as I press the button at the beginning of the race?

Just wondering if anyone knew!