Since I turned into a Strava-b*tch, I realised my rides to be mostly on the short side. I have my Edge 810 set on autopause at the lowest possible setting (think it's 0.1 km/h), but now and then during HAB it will slip into pause, only to slip back to resume shortly after. With this my HT550 for example shows a meager 850 km, but then, not quite sure it's the true reason.
When using the mapping tool (schweizmobil) - which I believe is very accurate - it actually does line up pretty nicely, my Edge showing mostly a slightly longer trip (which is good as a GPX track in theory always will be a bit shorter than what's covered on ground).

Some thing I'm missing on Strava?

How do you handle autopause? Do you even use it, or just go manually?

It used to be a problem when Strava started but their code now automatically handles stops so you don't need to set it on the Garmin. It's why you get "moving time" and "elapsed time" for an activity.

i have it turned off, doesn't save any battery life. strava works it out anyway.

I have Auto-Pause set to "when stopped". If I were to find it pausing during long hike-a-bike sections, I'd then just change the setting to "Off".

Cheers for the answers, will give it a try in manual mode.