Some drives, like the vertex 4/octane and samsung, record whole erase blocks bad when they are found.

Other drives, like the sandforce drives, only record individual pages when they go bad ... for example, on the Intel 330, each page is 8kb, so when a write/erase error occurs on a block, and the page is marked bad, the bad block count increase by 2 (as each page is 2x4kb blocks)