It makes perfect sense, really, for Word for iPad to rank higher than Excel and PowerPoint, considering the number of people who make use of Word for iPad a whole lot more than the other two productivity apps. It must be noted, however, that as of early Monday, the mentioned rankings had already dropped a wee bit, with Word for iPad dropping two places to no. 8, while Excel and PowerPoint hit 16th and 40th, respectively.
Apple has a policy that involves not disclosing the dollar amounts for iPad gross of its apps to the public, but basically ranks them for your perusal. It is a figure that only Apple and the app developer (Microsoft in this case) would be able to have access to the figures.