Google updates their rankings about 500 times a year. Yes, 500!
Webmaster Tools does not tap directly into Google's back end data - the live ranking goodies. Insteaad WMT is a secondary reporting system and it can lag quite a bit behind what you see in the live SERPs.
it updates it search algorithm 500 times a year but not it results... it tweaks it to give better results but there is no way their computer can compare all the sites for every search in the world almost twice a day.
My guess is that it is much slower than that, it has to first crawl the site they put the new information in it system ( let's say the links and links structure ) then it need to compare it to other sites and then re-rank everything, to me it is done once a month based on the popularity of the page and site it is trying to rank...