App server throughput and Browser throughput are measured in different ways: Requests Per Minute (RPM) and Page Views Per Minute (PPM) respectively.
App server transactions (RPM) will generally be larger in number than Browser transactions (PPM), somewhere between 2x and 11x app server transactions:browser transactions depending on the site. The largest reason for this is that API usage and AJAX calls fall into RPM instead of PPM.
Additionally, some pages of you application may not be instrumented for Page Load Timing, some requests may come from old browsers or browsers that aren't fully functional (API calls, RSS readers, indexers, etc). Pages may be also be served with a content-type that isn't text/html, or users may be navigating away before the the page finishes loading, which would both lower the app's PPM.
Back to your bullets, each of these are based off of Page Views Per Minute. Specifically:
Throughput by browser graphs the page views for the selected time window for the "big three" and "other" browsers.
Throughput per page graphs your app's top five viewed pages for the time window.
Top browsers by throughput is a combination of the other two. It shows the top five page view generating browsers and a sixth "other" category.