Why do you say it's unbelievable?
You have witnessed it often enough to know it's true.
Most people cannot add up - so why would you expect them to be able to compute the weighted average of the speed travelled over various segments of their journey between two points (whilst checking Facebook, obvz)? I would bet that only 10% of the population could compute that with confidence.
The cost of being caught is high. So for the 90% who cannot be sure, why wouldn't you expect them to reduce speed to significantly below target in the final segment to compensate for any accidental speeding (including speeding you weren't aware of because you were checking Facebook).
The psychology, maths & observed evidence all point the same way.
So I would argue it is totally believable and an inevitable result of a stupid scheme dreamt up by some control freak (no doubt in the 10% that can do sums).
Also, for the 10% that can compute the weighted average, they are also able to work out that you cannot be caught if the first camera didn't see you - so you can tear along at any speed you like until the next gantry - hence speeding along and then slamming on the brakes at the first measurement point.
My favourite is to weave between the lanes, hide up the bumper of lorries, etc.
As for Gatsos the same is true - why travel at an average speed of 76 when you can do 90 with short bursts of 65?
Cheers,
Robin