Basically, it's for efficiency reasons. Some of our sites have well over 40 pageviews per second and updating the numbers that frequently would put a huge amount of load on our servers and also crash the browser or make it go incredibly slow.

We also like to use the very same API that's available to our users in the Now and Trends dashboards as it encourages us to ensure response times are quick and that everything we track and display can be used programatically in custom integrations (such as the trending section on http://carthrottle.com).

Hope that answers things :)