ESPN/Hoopgurlz rankings are done by Dan Olson, of Collegiate Girls Basketball Report (CGBR).
Olson updates the CGBR rankings almost weekly, based on evaluations from tournaments he attends and players he sees. I tracked rankings for Duke commits Jaida Patrick and Jennifer Ezeh on CGBR over the summer, and their respective rankings changed at least five different times from June through August.
For the last 5 years, ESPN has published Olson's rankings 2-4 times per year (depending on the year). These are snapshot rankings, at different points in time (specifically, the date(s) of publication). CGBR is a subscription service with specific evaluations of several hundred players per class, whose rankings vary throughout the years (as I noted).