The Explanation

Floyd had two consecutive rest days, according to the Phonak doctor.

"On the 17th stage, he rode five hours with about 360 watts. In training
he could do that for eight hours. And you have to take into
consideration that he had more or less rested the day before." By that,
she meant that his poor performance the previous day was because he
hadn't eaten enough - "therefore he couldn't pedal with his full wattage".
http://www.cyclingnews.com/news.php?id=news/2006/jul06/jul30news

A full bonk is a rest day, apparently.