PowerMapper only collects pages that can be reached by clicking on a link, mainly the HREF attributes of A tags.
SortSite needs to extract a lot more links to perform its function. For example, some IMG tags have a LONGDESC attribute that points to an HTML page that is only accessed by screen readers (it's used to provide a narrative description of an image for low vision users).
In addition, SortSite also counts CSS, JavaScript, Flash, Video, audio and images in the total page count on the dashboard.