I am currently towards the end of the Base Period of the 24 week Mountaineering Plan – first time using this plan though I have done a few cycles of self written plans over the past few years.
I have done a few of the “Steep Uphill Hike Zone 3” workouts and have noticed that my HR is considerably lower than I would expect for the perceived effort – mainly in the 140s, with an occaisional spike to 155 or so.  It feels like a Zone 3 effort for sure but I have estimated my AeT at 158 from nose breathing test.  I have done a few AeT tests over the last couple of years, with slight improvements, and confident the number is about right, and a long run with a HR in the 140s feels pretty cruisey in comparison to this.

I realise that it may just be due to errors in the HR monitor, but it has happened on a couple of workouts. I use a chest strap and I have checked my HR manually a number of times during the workout and the numbers correlate well enough.

I am doing these workouts in boots, with just a hydration pack, on the best I have access to at the moment which is laps on a nearby hill.  The route is a mix of steep fall-line and flatter sections of trail, 220 metres of vert in about 1.1km and taking about 15 minutes up.  My HR drops to under 100 on the quick hike back down for the next lap.

Is it likely to just be a HRM issue? Or is this variation somewhat normal?  And due to the relatively less efficient form of locomotion?  Or am I missing something / erred in my training along the way?

I did look through the forum archive, so I apologise if I missed a previous discussion on this.
Thanks for any insight.