That's one of the interesting opportunities of the Steem Blockchain, is that other people/companies can create interfaces to display content. So while Steemit is currently the biggest, and was created by the same team that built the blockchain, it was really just meant to kickstart things and act as a first entry point. Over time, hopefully more sites will allow people to create accounts on their site instead of Steemit. Agree that it can be very confusing though, because you don't immediately realize that your same content could be showing up on a bunch of different websites.