my half-assed guess would be the optimization of power production vs CdA. Leipheimer is a small guy, maybe 135 lbs dripping wet. He needs to minimize his CdA, and if he pays by losing 20 W, so be it. Cancellara has 45 lbs on Levi, and assuming his ftp is 6.4w/kg (which is what world record holders put out), that's 280 watts right there. In his case, it may be better to get more watts out and pay a little more price in CdA. That scrunched-up position of Leipheimer sure doesn't look very comfortable.
Edit: got ninjaed thrice over...