Would anyone help me understand why the zero setting would change after a year?

After a year I needed to adjust finer. That would be good for the next shot. Then it would be too coarse then it would have to be set finer again etc until it was at zero.

I have not yet taken the hopper off to readjust the zero setting, so I do not know if the two set screws have loosened.

edit:

Anyone have difficulty removing the hopper?

This one is as if it is cemented in.

Next step is to wrap the collar with duct tape and try with a large pliers.

Any suggestions?