its not that inaccurate. Most of these estiamtions occur in low km speed zones where it is quite obvious when someone is speeding.
Also cops won't accuse you for only speeding 10kms over as it's to close to judge. From what I've seen and from talking to a few cops who are friends they generally only use judgements if it's blatantly obvious you were speeding.
"Yet on the other hand i believe they can use some descrepency, as another time i got caught street racing, but let me off on a speeding fine. They told me that they assumed i was doing over 100km as they had to do 130km to catch up to me. I can't remember if i got booked for 110 or just 100, but they were nice and let me off on less serious charges due to some circumstances."
Uhmm yeah, cops have always had the power to issue warnings and use their own discretion on certain matters. If you were street racing though I'm surprised you didn't get the book thrown at you. keep it on the track.