Power slightly too low in ERG mode

When doing workouts in ERG mode, I’ve noticed some puzzling behaviour and I can’t quite determine if it is a feature or a bug…

My setup is a Tacx Vortex smart trainer connected to my Macbook Pro using Bluetooth (and the Companion app on Android). I calibrate the trainer on a regular basis with the Tacx app and keep my tire well inflated.

The issue is that often the power recorded is ever so slightly (a few Watts) lower than the target. For example, yesterday I did a workout with some 10-minute intervals at 135W, 85 rpm. I settled in to a steady 85-86 rpm cadence, and the power bounced between 131-133 W but never actually reached the target 135 W.

What’s even stranger is that my trainer is certainly capable of quickly zeroing in on the correct power, because it hits the target numbers dead-on during warm-up and cool-down ramps. So why is it failing to hit the target during the workout? I’ve also noticed that the problem is the worst in the Tempo (green) zone. I can’t tell whether it’s me, a “feature” of the workout mode, a wonky calibration curve on the trainer, or something else…

This is driving me absolutely crazy since I spend entire workouts with the target power just barely out of reach!