I've thought about this a lot, too. For me, I think it's a combination of a couple things:
- needing to use the map to know what direction to go
- soft underfoot, so no rebound from the ground
- branches and blueberries and stuff that you have to push against
- mini direction-changes to avoid said branches and stuff
I've also noticed that even running on a trail I'd consider "hilly", orienteering always collects more climb according to my watch. So, that adds to it too.
That is a good point, my most recent hilly run had less climb. But the pace that AP calculates that's prorated for climb? I was going a lot faster on the trail run. I agree that it's surely a combination of factors, I don't disagree with any of the above. I ought to be able to do better, though. I think.
Heart rate usually reflects that even when walking on an O course, I'm still working pretty hard aerobically, whereas I really need to push it on a hilly trail run to maintain the same HR. So it seems to me to be more physical off trail rather than the mental aspect of things.