It was something do with, never driving at exactly the marked speed limit (particularly at night) as any following police officer would be suspicious? How true this is I don't know.
Police officers are most likely going to pull someone driving slowly at night, as they're probably under the influence of something (or excessively tired) and think driving slow will make them invisible/safe.
In any case, anyone driving at what they believe to be the limit will be driving too slow anyway. All speedos show a higher speed than being driven, so if a police car is a traffic car with a calibrated speedo - they'll see you doing 3-4mph under the limit anyway.
When I used to live in London and not far from the police station, local cars at night (as in 1-4am) would often pull over to let me pass and didn't have a problem with me possibly doing slightly above 30mph. They were on patrol and didn't want to hold me up.
I also overtake police cars on motorways and don't just sit behind when they're in lane 1 doing 60mph and forming a giant convoy of people who are too scared to overtake. You'll see it all the time. The police have a term for it but I forget now (must ask my friend who's a traffic cop one day, as they always find it amusing).