Turbo trainer overestimating speed
I would expect that riding in a trainer would result in a different average measured 'virtual' speed than you would see riding on the road. The speed/resistance curve of the trainer will not match the actual speed/resistance curve you experience riding on flat ground in real life.

In any case, I don't think the virtual speed actually matters. What you would pay attention to is the estimated power. Unfortunately it seems like these trainers have power estimation problems.

http://www.olympum.com/sports/the-curious-case-of-mr-elite-turbo-muin/