Don't rely only on gpsr, next time try getting another waypoints, like a singulaar tree, a rock, something like that.
You can try digging a 20 ft hole, after all, the error could be bigger!
Did you checked the EPE before hiding it?
If Estimated Predictable Error (EPE for short) was high, you should expect that kind of variation.
I advise you on using WAAS, the sportrak is good, but a little help never hurted.
Good luck!