You can read about bank-switched memory here:
https://en.wikipedia.org/wiki/Bank_switching[
^].
Your arithmetic concerns hardly can be explained based on just two sentences taken out of context. Such sentences can always be understood incorrectly, and books can contain some error. No matter what it is, you can always find out the real sizes of banks and other units from other sources, in particular, original Nvidia documentation. Fermi microarchitecture is explained here:
https://en.wikipedia.org/wiki/Fermi_%28microarchitecture%29[
^].
—SA