Nicola,

Analytics is rife with such minor errors. For Time On Page, it's the Exit Percentage that matters. Any page with a 100% exit percentage will show 0:00 Time On Page. It's also important to note that when Google computes Time On Site, they average in all reported page views, including those that were single page views resulting in 0:00 for the visit time. But when they compute the Time On Page for an individual page, they throw out the zero values as soon as they get a positive value, and then they only average the number of pages that had positive times. Wrote it all up this morning for my blog after running experiments all yesterday:

http://www.fonerbooks.com/selfpublishing/?p=55

Morris