I had similar experience years ago. Lying on the ground using the bi-pod sniping rabbits out to 300, couldn't miss. Tried using off a bench at the range at a 300m fox target blew the group out to about 8 inches. Put the rifle on proper front rest and the group came back to under half that. My thoughts at the time were that the rubber feet gripped the bench and tended to make the rifle jump under recoil rather than smoothly travelling straight back as it does off a proper rest.
Rifle was .257 Roberts shooting 100gr bullets @ 3300f/s so there was a bit of recoil. This was my hunting rifle at the time and I had thousands of shots at game off-hand, prone, resting on logs, rocks, out of a vehicle spot-lighting as well a using the bi-pod in the paddock and never noticed any change in point of impact until I tried it that once on the bench.
I'd better add that I had only borrowed it to try out, didn't bother buying one.