How old is the house?
Keep in mind that anyone could have done electrical work on the house including past homeowners, so no telling what you will find, especially in old homes. Also other workers come along and do things.
Anyway it could be that someone at one time might have connected a cold water pipe ground as well as ground rod(s) outside. Then maybe a plumber (or homeowner) needed to fix the sink and removed the connection from the cold water pipe and never reconnected it.
Or maybe metal water pipe was replaced with plastic pipe and it was pointless to keep a connection to the cold water pipe.
Or maybe an electronics type had a ham radio and wanted a good ground inside and had a wire run from there to his ham radio.
Or maybe it is an old house with two wires and no ground and someone wanted to ground the garbage disposal or dishwasher.
All sorts of possibilities!