[r9etb]

To tell you the truth, it sounds like a simple miscalculation in their math. If they simply add the extra 13mm per second, the unexpected acceleration goes away.

It's not so much a miscalculation, as something missing in their perturbation models. I don't know if they account for relativity effects, for example -- but if they're not, they'll see an error.

It all boils down to the source of the error. It might be nothing more complicated than trajectory determination error.

[RobRoy]

Maybe something here could explain it:

“Why Gravity is Really Pushy:
http://books.google.com/books?id=8pR50SFD4YAC&pg=PA134&lpg=PA134&dq=why+%22gravity+is+really+pushy%22&source=bl&ots=vNgGsSwEvi&sig=L9HX2nqHHW3f5iKabzGPsSo32BU&hl=en&ei=pOs8TJ2EA4ycsQOR_p3aCg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBIQ6AEwAA#v=onepage&q=why%20%22gravity%20is%20really%20pushy%22&f=false