Could it be the result of Google lobbying for underestimating their search market share (due to antitrust concerns) and Yahoo/Bing are lobbying for overestimating their market share?
I don't think so (but your thought is very interesting anyway) because when Bing lobbies to get higher stats, they may get higher ads revenue but in the long run, particularly in the FTC investigation, they will run into bigger trouble.
But yeah, this is just my guess...