Class is permanent, form is.............puzzling

Evening All

Can anybody explain the "form" number to me? Tom Veelers has 0.0 form despite two top five finishes, whilst David Zabriskie is top of the domestique form chart despite being invisible so far in this Tour. I'm happy to accept that it's possibly a figure based on more than the last week, but the last week would be more helpful when selecting domestiques who's names are less than household.