Ok, that answers the why they get hours faster. I actually thought of that after I posted.
the general figure around here is that if you can run a machine for 500 hours a year, you can make money with it. (It goes to the site, it comes back to the shop. 2 hours gone in a day. It gets used 3 days a week. 18 hours a week. Your operator takes two weeks off in July and August. 50 hours a month. Fall to snowfall it sits idle. Snowmelt to spring it sits idle So it only runs 8 months a year.)
But what wears out? Do hydraulic power systems have an intrinically shorter life span than a mechanical power system?