For what little it's worth, my dash hour meter and odometer do not agree with ECM. The odometer is off by a few hundred miles and the hour meter is off by a dozen or so hours. Sorry but I don't remember which direction in either case.
This was revealed when I installed a Silverleaf VMSPC. The VMSPC takes its data from the ECM. I've never attempted to adjust the dash gauges to reconcile with the Silverleaf and would not have any idea how to do so.
The Silverleaf does have an offset function which allows one to adjust its readout to reconcile with the dash gauge, however. The respective units of time and distance rate of accumulation can also be adjusted to keep the two readouts in sync.
The ECM data is the true and correct representation. That said, I don't know what the situation would be if the ECM had to be replaced.
Good luck in resolving the discrepancy.
Dave & Cathy, 2002 Country Coach Affinity, 42', #6103, CAT C12, 2003 Cherokee Overland