You pretty much nailed it, but one additional factor is that application "bundles" in OS X frequently include all the required libraries required to run the app

You pretty much nailed it, but one additional factor is that application "bundles" in OS X frequently include all the required libraries required to run the application, rather than linking against shared libraries. The only shared libs you can really count on are the ones provided by the OS, so if you need something newer, or different, many app developers simply bundle the library with their application.