Power consistently low driving trainer in ERG mode

I'm pretty sure this has been asked before but the answer doesn't make any sense to me. I thought I would ask one more time before wasting money on TrainerRoad.

I have structured workouts written in TrainingPeaks and connected to my Garmin account. I've been doing this for years and it works great for riding outside. However, when I use my Edge 1030 Plus to drive my Wahoo trainer in ERG mode, the power is consistently too low. In the attached picture, I'm riding in a steady state for a long time. Cadence is a consistent 90. The trainer was calibrated through the Edge before starting the workout. It is always like this, every time I have tried to use it for an indoor workout.

(The drop in power at the end is me stopping so that I could take a steady picture)

  

Here is another way to look at it. The Edge was telling the trainer to set ERG mode at 181 watts, but all it was getting was 159 watts. Again, I was riding as steady as possible and it had been at that power level for several minutes.

Do I really have to pay for a software subscription just to ride a simple workout indoors?