Would you happen to have a copy of the original NIP you could show us? I've lost count of the number of people who've told me they've been caught at 31/32 in a 30, so far not one of them has been able to show any proof.
It is because it doesn't happen. No one is going to try and do you for 1mph over the speed limit because they know full well that they could not prove beyond all reasonable doubt you were speeding. That is why 10% + 2mph guidelines are used because whether you argue there is a small margin for error or not, you will still have been speeding.