Distance is always off

The Edge 540 seems to always have problems with distance??

I group ride with 10-15 people including my husband who rides with an 840 and he always has 1-5km more distance than I do on the same ride…the longer we ride the more it’s off. 
The group tonight everyone had at least .5 - .8k more than I had for the same route. 

How do I fix this problem?