[zog]

--my completely unsubstantiated guess is non-precise calculations, bad data by a small degree, leading to erroneous conclusions.

second guess, totally different. correct conclusions, reason for slow down is from the presence of onboard radio transmitters, the propogation itself, they are somehow tied in with gravity.

/layman's guesses

[Ken H]

I hold with the "what goes up must come down" explanation.

It's clear and simple, and can be understood by the people.

[El Gato]

my completely unsubstantiated guess is non-precise calculations, bad data by a small degree, leading to erroneous conclusions

If it were just the one probe exhibiting the anomolous accelleration, you could be right. However it's all of them, AFAIK. Such errors would tend to be in different directions for different probes, with some appearing to not slow down enough, rather than too much. With all the "errors" in one direction, it's probably a real effect.