That's not as secure (or at least not as obviously secure) as when we mark the paper ourselves. People make mistakes, and people miss errors. This creates roo

That's not as secure (or at least not as obviously secure) as when we mark the paper ourselves. People make mistakes, and people miss errors. This creates room for the software to introduce rare (but, in some races, common enough to matter) errors in a particular direction and rely on them 1) often being missed, and 2) accounted to user error when noticed.