when your server runs your card, he only inputs the amount of the check, which is what is approved at that point. After you sign the ticket and include a tip, the server then goes and adjusts the original check amount to include any tips. If you did not include a tip, there would be no reason for them to readjust your purchase. Did you save your receipt?

Unfortunately, there are some servers that if you pay by credit card and do not leave a tip, they will add a tip themselves. So it is never a smart idea to stiff a server if paying by credit card, even tho it is illegal for the server to add tips, and if he doesn't get caught, he's gonna keep on doing it.