Possibly because analogue speedos are notoriously inaccurate. If the machine does only do 75-ish, then it´s just your speedo is reading 30% faster than you are really going. It wouldn´t notice so much at slower speeds, as the percentage is the same. It is the same with cars.... never trust an analogue speedo! Could be worse... could be reading you slow... at least you won´t fall foul of a GATSO for the sake of 3mph!