I also think a BMW website error is the most likely, but since the MPG ratings haven't changed going from 2008 to 2009 (according the BMW website), the only other classification is if the comparison vehicle/class has been altered by the government, i.e., reclassifying the comparison. From what I have read, the credit is not determined by the absolute value of the MPG, but rather how much it improves over a comparable 2002 vehicle. e.g., if BMW had gotten away with comparing it to a V8 X5 when they made the 2008 request, perhaps the IRS has come back in 2009 and said, nope, the actual comparison is the I6 X5, we made a mistake with your 2008 request.

I'm not saying this is what happened, but IF (Big IF) the tax credit has truly been reduced, that might be the most likely explanation.

One of these days the IRS website will be updated and we'll know for sure.