Hi ​@Nico Ns 

This depends on many factors - what devices you're using to record the activity & how, where (open area vs. tunnels & very high buildings), environmental conditions (very thick layer of rain clouds, ...)

This article is a good place to start:

https://support.strava.com/hc/en-us/articles/216917707-Bad-GPS-Data
 

If you want specific advice, you'll need to include more details, ideally with screenshots or a link to the activity. 

Here are some samples:  

One - should be 4 miles or so instead got 7 miles

Two - should be 4 miles as well but is 5.  I”m not walking around either once I get to work

I’ll add others once I find them on my phone.

These are really weird cases. The GPS looks good, all the graphs in the web UI show the expected shorter distance but the distance in the summary is completely off. It would be really interesting if others get similar inconsistent results when recording with the Strava Android app.

Hmm… interestingly, when I go to the Analysis tab of either of the activities, it seems to show the correct distance (around 7 kilometres, so it's about the 4 miles). 

It seems like the file got somehow broken If you try the “correct distance” feature (on the web version only, not in the app), does it result in a more realistic number?

I corrected the distance but it showed me the same mileage.