Why do certain websites (including Similar Worlds) falsely accuse different people of being the same person when I know they are not?

Logical answers only, please. Note that "Using the same IP" is not a logical answer because I am the manager of an Internet café and lots of people use my premises to access various sites. I don't see why the site admin can't accept that. So remember, just because you've detected a certain computer being used, you have no way of seeing who is using it, so please stop these accusations.