actually the file free has more to do with the device features. both devices are 64mb memory devices as JS said, and its not all the extra carrier branding junk, its actually the video recorder, wifi browser, voice recorder, etc
I guess the best way to explain this is to compare it to the HDD of a PC. Say you have a package listing of 160 GB HDD on your laptop. If you go into "My Computer" (and other options) it will generally show 8-10GBs less. Simply put, there are certain OS files and applications from manufacturer that use a portion of the HDD that you can't free up. I understood what was posted above but maybe this explains it more simple.