... even the digital camera have internal memory. It is used as the primary storage when you take the photos. After that the photos are written to the Memory card. This is why even professional cameras have a limit for fps ( depends on the internal memory ). And sometimes you have to wait some time till the photos are written to the card before being able to take more photos (there are cards faster then others - CLASS 8, 10, etc).
Mental note : I have to by a class 10 card for the garmin!!!
This is why I was "guessing" the increase in the internal memory.
The "t" in the reference also increases the amount of memory, so I would "guess" that topographic maps also need more internal memory ( like a video card on a PC ).
One thing that I have found, is that different Garmin sites ( different countries ) have different values for the internal memory ( specially the 600t model).