Hey @Crazykeef, 
Thanks for your post and sorry about the discrepancies between your activities as shown on another platform and Strava.
 
I'm not sure how that platform calculates distance while recording but the file that is uploaded to Strava does not have a distance stream or any predetermined distance data, it has GPS data that can be used to calculate distance. When we calculate the distance based on GPS data, we end up with the distance you see on Strava. There is no other way we can interpret that data. It's possible that the device itself is using a different process but it's not sharing that information with us. Unfortunately there won't be any way for us to process that data any differently or edit the distance on Strava.
 
For more information, see our article on How distance is calculated for your activity. I hope that helps. 

I have recently experienced the same problem with run data imported from Fitbit. I have a route I often run that is 2.5 miles. Fitbit consistently records it at 2.5 miles. Strava will normally record it at 2.5, but most recently recorded it at just 1.9 miles. A similar issue happened earlier this month. The map on Strava correctly show that I run the full route, so the problem is not with the GPS data. Strava obvioulsy has all the correct GPS data in order to generate a correct map. That means the problem is clearly in how Strava calculates distance based on that GPS data and this needs to be addressed by Strava.

I have the same issue as the original post. The Strava map is the same as the Fitbit map yet on the 3.5 mile run Strava displays 2.33 miles. How can you have the GPS data to draw the map but not measure the distance?

Hey @Ged & @onealjd, 
Thanks for your posts. If you would like our support team to take a closer look at your activities, please submit a support ticket and our team will investigate further. Be sure to include a link to the activity/activities in question as well as the device you are using in your support ticket.  

Is there already some progress on this topic? I am facing the same issue since this weekend, a 16km Fitbit run was 'only' 14km on Strava. And this morning a 10km Fitbit run was translated on Strava as not even 8km. Both should have been personal records on the 10 miles and 10 km so very disappointed that things went wrong with syncing.

With me the maps also look the same on Fitbit and Strava, but Strava concludes the distance is a significant amount shorter.

Dived into the details, see the parts I annotated with the blue oval around it. Looks like this part is just simply ignored by Strava. According to Strava my run just simply ended sooner while it seems like the data is there.

The activity from this morning can be found here: https://www.strava.com/activities/11259090659

I eventually established that this is a Fitbit/Goggle issue rather than Strava. Is you have the same issue I had, when you view the activity on the Fitbit app you will see a GPS map under "Heart Rate Zone". You will also see a GPS map under "Pace". In my case these maps where different, with the sections of the run missing, or ended early on the pace map. I have attached screenshots.

This is despite the activity only being recorded on the Sense 2, and the watch showing me the completed distance and time when I fished the run. I think Strava takes the info from the Pace section in Fitbit files and therefore recorded the incomplete activity. After several issues with Fitbit/Google being unable to even recognise this as an issue and then recurring syncing issues between the watch and my phone I went back to my trusty Garmin. 

Strava did respond to a trouble ticket and a work around on there side to display the full activity, this response is below and it might help you record your PB!

Strava response:

In some situations you can change the distance that is shown for your activity. Changing the distance stream may also update the pace or duration of an activity. If your device created a distance stream using sensors such as a speed sensor, pedometer, or accelerometer, you can switch your distance on Strava to use GPS data rather than the generated distance stream.

To swap your distance stream:

Go to your activity page on the website (this feature is currently not accessible in our mobile apps)

From the activity page click on (◦◦◦) the more options menu > Correct Distance.

From the pop-up, click "Correct Distance".

When the status changes from "Calculating" to "Updated," refresh the page.

If you would like to revert the change, you can click the same link and click "Revert Distance".

I have gone ahead and switched the distance stream for the activity mentioned above. You should now see the correct distance reflected there.

Thanks for taking the time to clearing this out for me. I seem to have the same problem indeed. Correcting the distance brought me just below 10km and just below 10 miles for my weekend run. So I thank Fitbit (/Google) very much and will also look for a better alternative...

and all the kudos towards Strava (and this community)!