Power meter not matching Tacx

Garmin 925 “watching” my CyclOps SL+ hub-based PM.

Tacx Vortec Smart trainer, connected via BT to a Mac Mini.

HR and GSC10 (for cadence) via ANT+

First group workout tonight.  Selected the Tacx as my Controllable trainer, and I THINK I had checked the ERG box on setup, but I’m really not sure.  Joining events seems different than the Workout selection screen.

I had the Garmin display my PM on 3s average, while the Watts from the Tacx were displayed on the TV screen.  During the main intervals, I was to hit 160W for 12 min.  I held cadence, and the trainer did exactly what it should.  But the Garmin was showing 140W (which would be the CyclOps output).  

At the end of the workout, Garmin Connect sync’d with Training Peaks, and I had 2 workouts to compare side-by-side.  The Zwift export to Training Peaks had a 150W average for the hour, and the Garmin Connect one was 140W.  

I ran a quick calibration on the trainer when I was done, and it was spot on.  I’m NOT looking for perfection here.  In fact, as long as the trainer is consistent, I can swap to an old wheel with a trainer tire.  Is the easiest route to simply do an indoor FTP test with the SMART trainer and use that?  I had done the test with my PM and if that number is off, then I’m not getting the workout I would like when doing the “E” group rides.

Cheers,

Michael D.

Oh, and worth noting that my TSS score was quite different on the 2 versions of the workout.  This is a more important datapoint for me than my indoor vs. outdoor FTP…