Given that most of the people involved in the checking of your signature aren't exactly trained in forensic document analysis, it'd be pretty easy to claim 'mismatch' in a very large number of cases.

In practice, they're barely even looked at. There are various signature-trolling stories around the net (signing as mickey mouse, Hitler, random pictograms, etc) demonstrating this.

When I worked (thankfully briefly) in retail, I'd give every one at least a cursory check, and maybe a bit more detail and less tolerance of differences if it was a big-ticket item. I actually got chewed out by my boss occasionally for checking, since few of the other staff did, and it might "upset the customer".