<takes a deep breath>
The ROM you are talking about is actually the program inside a SMB cartridge. It was copied onto a disk or hard drive by a special piece of hardware, which read the cart just like any other removable media and made a duplicate. These machines are called 'backup devices', 'game copiers', etc. and are the reason why the emulation 'scene' exists.
Sometimes, these copiers are not able to completely and perfectly copy the game program. This is usually due to special chips inside the cart itself. Whenever a cart is not copied perfectly, a bad checksum (or 'bad dump') results.
With a first generation title like SMB, I can't imagine there being any difficulty making a perfect copy (there are no special chips). If the ROM you have is a bad dump, it is most likely a hacked copy.
