Charge 3 and Fitbit App Distances are Different

[Mom2PCC]

I run the same route every time I run. I've been having trouble with inconsistent distances. I just started running and am trying to meet a goal at the 3 mile mark, but can't figure out where the 3 mile mark is! 

Everything is updated, and I restarted both my phone and my Charge 3 recently.

I was originally starting my run on the Fitbit app on my phone (waiting for GPS to determine my location on the map before starting the run). When I did that, my Charge 3 would also start. I'd get the voice updates every .5 miles (which I want). At the end of my run, the app would have recorded 2 runs- one from the Charge 3 (the one that lists heart rate), and one from the Fitbit app. The times are the same, but the distances are .3 miles different. My Charge 3 said I went 3.39 miles and my app said I went 3.01 miles. Same run, same day, same times. Both maps look the same, with no gaps in GPS. I have a photo of it, but I tried for a long time and can't figure out how to insert the photo into this post :(.

On today's run, I decided to try starting my run from my Charge 3 instead of through the app. I made sure to wait until the GPS was connected before I hit start. I did not get the voice updates, even though it's turned on as an option in the app. The distance was the same as the Charge 3 distance the previous day, which was around .3 less than the app. (My Charge 3 told me I was at 3 miles about .3 miles before when my app told me I was at the end on the previous day).

Don't the Charge 3 device and the app both use the GPS on my phone to figure out my distance? If so, why would those distances differ on the same run on the same day? It makes it hard to set goals and determine if I'm meeting those goals or not.

I've seen others say that stride length needs to be put in manually, but if the distance is determined by the GPS on the phone, this shouldn't matter...nor would it account for a difference in distance between the device and the Fitbit app.