Forgot to add: by repeated formats, you lower the actual values until they disappear into the noise floor -- a "5" is pretty hard to tell from a "4", and a repeatedly rewritten 0 might go to a 5, whereas a repeatedly rewritten 1 might go to a 4, so at some point entropy takes over.
If you've used SpinRite, you can sort of see this in the display -- the values aren't all high or all low.
