When Is Ten Hours Not Ten Hours?

When is 10 hours not 10 hours? I found that out this morning when I paid for 10 hours parking and the machine only provided me with nine hours and 51 minutes. I had quite clearly paid the correct amount but been diddled. It would have been most interesting had a parking inspector nabbed me within that nine minute period as I feel that I'd have a case in court to defend myself. Or do you have a case when you are the litigant and not the defendant?

Maybe I should look a little more closely when I have to pay for parking that I'd not being short changed.