How come our apple watches show different distances

How are we supposed to trust the Apple Watch fitness and activity apps when there are such huge differneces of measures.

My wife and I set off together on a walk. Having both just got up that morning and put on our watches.

Yet my watch records 5.2 miles and hers records 4.7 miles
We walked exactly the same distance.
This is a permanent issue. How can we change and calibrate them to record correctly.

Lev.