Memory is partitioned into B&N space, and User space...
You could have the B&N space full, while the User space is empty.
If you go into the shop (I'm interpolating from my HD+) and pick some item, does it (details) show the size of the item and the size of the "available space"?
I think I've read that the default B&N content is limited to 2GB
my BN Store memory is what has 24% left, other storage is empty and I have all of that memory.