It may very well be that changing the UI map is the easiest way to accomplish this. Any particular person may not have access to the backend code, but may have

It may very well be that changing the UI map is the easiest way to accomplish this. Any particular person may not have access to the backend code, but may have access to the code that maps screen positions to selection. Having no knowledge of this machine in particular, I can't say much more, but it's certainly feasible that this is the easiest or only method to modify the outcome of a vote. Based on the description and assuming it's accurate, we could conclude that there is some sort of mapping of the display to vote selection that is configurable locally. This makes sense, as every locality has different ballots - school districts, etc. We do know that these things tend to be pretty poorly written, so you also can't assume even reasonable or typical component design. I don't think your logic is valid.