Old Smokes, I am really impressed.
Rainboe, I am disappointed at your patronising attitude in 2007.
Paris, all I can suggest is a combination of Altitude keeping error and Air Data Computer error acting in unison to compound the error.
Guess if you were in RVSM airspace you would have been twitching a bit more (like me transiting the Indian Sub-Continent & Mid East!!) So Good!