@aristotle: We don't know the specifics of how Panda works nor what data it relies upon (nor will we find out this information). Hence it's difficult to know *why* the Panda update can't be done (say) daily. In answer to your questions:
1) Not sure.
2) We'll never know. Panda probably collects and uses some fairly sophisticated data, hence it's probably this data that can't be updated daily.
3) It seems, re-run the Panda algorithm. For whatever reason (which again, we'll probably never know), Panda apparently can't be updated daily. Hence instead Google will simply re-run the Panda algorithm(s) periodically instead.