Exactly! So if the average is just below 70mph and we know some go much less then there are some going much higher.
Yes, obviously. But it does tend to indicate there are probably more people driving under the limit than over it - which is also what the 45% of cars exceeding the limit says (and 8% over it by 10mph or more).
Your initial statement was:
"Ever tried driving at 70mph on the quieter sections of the M6?
Okay, not 25% but that's only because it is a high bar. Excessive speeding is the norm."
My anecdotal experience, and the official statistics from the link provided by Ming say that no, excessive speeding is not the norm.
Anyway....The golden rule is/was +10%+2. So for 20mph that is 24mph, or +20%. An extra 1mph to give 25% is hardly a huge extreme.
That "golden rule" is the speed at which you will usually be "done" for speeding. So yes, they are expecting the average speed to be above the level where people will be getting speeding tickets. And the expectation was 26, not 25 - I've been saying 25% because I couldn't be bothered with the fractions.