Sometimes my Garmin is a jerk

I love my Garmin, but sometimes it's a jerk. 

I'm sure there's an explanation for this troubling behavior.

Today's 'speed workout' done on a treadmill, GPS off, and the watch using the accelerometer.  Which is woefully inaccurate for me.

Workout was
Warm up - 9:00
9X 1:00 hard, 2:00 easy.  Easy past was 4.2mph.  'Hard' pace for me is 7.1mph, which was really hard and felt like an accomplishment.  Only Garmin thinks 'hard' is about 6mph.  Jerk.

Garmin thinks my 'hard' pace is a lot slower than it actually was, and that my easy pace is much faster than it actually was. I'm choosing to believe the accelerometer is confused because my stride is so efficient, but I know that's probably a load of bull.  Sigh.