I’m not familiar with the trainer you are using but I know that many smart trainers will calculate a speed based either on the power you’re putting out or the fly wheel speed.
I don’t have a speed sensor on my bike but my rides in trainer road end up with speed/distance because the Kickr is broadcasting it as well.
Of course this is all garbage information in my opinion but it is definitely being broadcast directly by the trainer and not a separate speed sensor
I’ve never really looked for speed/distance in trainer road but my guess is that they are ignoring this data due to how arbitrary it is whereas Strava is processing everything that is sent to it - thus the discrepancy