Different reading between Fenix 5x and Edge 810

I have used my edge 810 as my indoor trainer for ages with the GPS turned off. This has served me well but I notice that my trading status on the Fenix 5x was always showing no training or detraining.

Decided to use the watch on indoor trainer setting just for some better data fields. I run both devices during a session and discard the Edge after the workout(I can’t look at the watch as easy while riding). I noticed that although they are both synced to the same sensors the cadence of the watch is 10-15 rpm behind the edge. I have tried to pedal to the watch cadence but getting it to 100rpm is so fast I can’t do it. It around 115+ on the edge

Just wanted to know if anyone has had such big difference in devices. And if there is any reason why they would be different.