The old paws aren't getting younger, we aren't climbing anything extreme these days. Jasmine turned 8 on this trip, actually set the new altitude record @ Bonney's, but the paws need a good rest now. Max is still pretty indestructible at 4 and just needs a good shampoo after wallowing in a $%^## dead fish up in the Titcombs.
GPS-wise, the old basic Garmins may be discarding any distance if your position changes less than the precision of the measurement (slow speed and/or poor satellite coverage). It's like you don't move at all then ... and the mileage comes short.
But model 60s have a different chip which is supposed to be a lot more precise, and I think that they changed the odometer algorithm. Now if you are slow (or if the reception is poor), it adds all the changes in your position, even the random errors! So the 60C may count a bit of mileage even as you take a rest.
The real number might be found by downloading the track, smoothing its random fluctuations and occasional holes, and re-measuring the smoothed track ... but I guess none of us are so serious GPS track aficionados. Just waiting for the others to do it