In Emulators / NES ROMs

For some reason ive been pondering this question. When people emulate games..how come sometimes there are "bad dumps" Does it not fully emulate all the way or something? Im just curious about it.

a "bad dump" means that the data from the cartridge didn't copy correctly for whatever reason when being dumped, and is therefore pretty useless. it may play properly, but graphics my be corrupt, or the game may be incomplete.