Its not random at all. From the New Scientist article:
"If it gains an extra 1.1 millimetres per second relative to Earth, it would vindicate a formula that reproduces the anomalies seen so far.
The formula, published in 2008 by ex-NASA scientist John Anderson and his team, hints that Earth's rotation may be distorting space-time more than expected and thus influencing nearby spacecraft, though no one can explain how."
Full article here:
The formula is given on Wikipedia.