Perhaps it's because I always charge with the power off?
Yup, that's why I fell into the trap of thinking that the value was total system elapsed time (and a product I've worked on had exactly that).
I suppose it would be possible to write a patch to prevent it resetting on charge. I don't know what the value gets used for but the fact that power-off charging for the H1x0 has the same effect without noticable problems suggests (but only 'suggests') that this might be useful.