Did you try to download the activity from Rouvy (training diary) as a fit file and upload it on garmin connect (web site)?
it would be great if Garmin connect would recognize these activities (without using the Edge or Forerunner) and the takes the data to Physio true up...
Unfortunately it does not work like this currently.
The True-up metrics are licensed from FirstBeat and implemented in the Edge or Forerunner devices, so an activity has to be recorded on those devices for it to work.
So for Zwift I have to dual record, Zwift sends the activity to Strava, but I have not enabled the link to Garmin. I record the activity on my Edge 1030 and save to GC to get the True-up. The speed and distance calculations are slightly different from the TACX NEO, but not like you are seeing.
This True-Up implementation is not ideal because of the dependencies on the devices, in my example I have an FR935 and Edge 1030 and I already see differences between them because Edge 1030 has latest implementation of FirstBeat metrics but FR935 not caught up - and I am not going out to buy a FR945 any time soon!
In an ideal world the FirstBeat metrics would be calculated as a cloud service in Garmin Connect so that any activity load into GC from any source could keep the one single view of your fitness up to date - but alas we are not there.
today I found a "solution": I deactivated the speed sensor on the Edge 1030 and had it just reading power, heart-rate etc... this worked since TSS, IF etc... are not function of speed but only power.
then I synched both, Rouvy and Garmin and just edited manually the distance on Garmin Connect, to have the whole picture. of course in GC I don't have the exact speed over the duration but for that I can see it in Rouvy, Strava etc.
on the other hand I have the "effort" on GC and since the activity was recorded by the Edge 1030 I have updated Training status, too.
a bit tricky but it works. but still strange that there is still this speed difference...
I have been looking at these variations. I wishes to record my workout on Garmin as it contributes to my weekly figures but had double the cadence and speed with Fulgaz and approx 10% more with Rouvy. I store my figure on Strava straight from the app. Them delete the Garmin workout when it is copied to Strava.The Garmin figure can be edited in the workout to correct them. Fiddly but as you say why are they different anyway, surely in the current world this issue can be resolved by Garmin. Do we assume the training platform is more accurate?