Power Discrepancy?

Are you seeing the variance as you ride or in the summary at the end?

I would troubleshoot in this order.

1.  Make sure both devices are recording power in the same way.  You want every second recording, not smart recording on the garmin.  I'm not sure what other options the Wahoo app has, but 1 second recording is always best for data integrity and comparison.

2.  Look at the files side by side without any smoothing.  If one head unit is reading 10-15 watts low, I would suspect that one unit is suffering from intermittent drop outs.  For example, if you are putting out 200 watts constantly for 10 seconds, the file should read 200 ten consecutive seconds and report an average of 200.  But if for one second the signal drops and reports a 0, then your average suddenly drops to 180.

3.  I am guessing that the wahoo app is connecting via bluetooth and the garmin must be using ANT+.  If this is the case, that's two different means of communication which could help explain #2.  It's not to say that both shouldn't work at the same time, but you may have some interference in the house (such as home wifi, apple tv, etc.) that is interfering with one signal and not the other.  You may want to try unplugging some of your other home devices to see if that helps pinpoint the issue.

4.  Try mounting or resting the head units closer or farther away from the Kickr to see if that helps explain #2 and #3.

5.  Borrow another head unit, phone, dongle, etc. and test that as another variable.  Maybe it's possible you just have a faulty head unit.