Maps accuracy during a activity is waaaay of.

When i´m in a activity and look at the map my gps tracking is way of, maybe by 100-200 meter. I have done two runs close to a lake. On both those times i were running on a big road. The map showed the gps tracke out in the water. Didnt even see the road. After when i look att my phone the garmin connect show me the exact location on the map. The gps tracking was then spot on my the meter.   

How can that be?