It's a similar reason to why athlete training for a 100m race runs a distance of 100 m over many attempts before taking the average.

You never know, the athlete runs 10 seconds in the first attempt, then 10.8 seconds in the third, then 11.1, 10.7, 11.1, 11.0 etc. Had we taken the first reading, we may be misled into thinking that the athlete can run in 10 seconds consistently.