Heart rate zones during workout

I've just started a heart rate based training plan (Improve your fitness level 1) but have a question regarding heart rate zones shown on the watch during the actual workout.

My heart rate zones are set up as in the attachment.


The first run for this plan is " Run in Z2, easy pace, 35 minutes." so I set of on the run and the watch started reporting "In desired zone" when the gauge hit the green between 2.0 and 3.0. During the run I kept the gauge as close to central as possible and this was 2.5. Now I had thought that at this point I was in the middle of zone 2 i.e. 117-137bpm but after reviewing the activity post run 2.5 actually refers to the middle of zone 3 i.e. 137-156bpm. I would have thought if the goal was to keep you in Z2 then the gauge would be from 1.5 to 2.5 not 2.0 to 3.0

My heart rate across the activity was pretty consistent around 145bpm so is this a Z2 or Z3 workout? I am confused.

Thanks ciq.forums.garmin.com/.../1263578.png