As an aside, your 31 in a 30 zone analogy isn't apt as there often an allowance for speedometers not being exact. And they rarely prosecute even at 35 in a 30 zone.
It's illegal for a speedo to underread so they would be perfectly able to prosecute someone caught doing 31 if they wished, but the guidance to the police is not to - perhaps for the reason that it could be seen as unfair.
PR wise it would be a nightmare, as I doubt most people would appreciate speedos are offset to avoid this problem and so someone caught doing 31 would likely be fully aware they were speeding as their actual speedo would be likely showing 34 or 35mph! If a speedo was underreading, you can also argue that this is of no concern to the police.
So, I think it was actually a very apt comparison and shows that the police use discretion all the time. Some for PR reasons, some to simply stop officers being dogged down with what would otherwise become a load more offences - as I suspect most people are quite knowingly speeding in 20, 30 and 40mph zones - but not to a level that anything other than a fixed, uncaring, speed camera might take an interest in.