They are all different ways of transliterating the sounds of Japanese names into English. It's hard to say which is more correct than the other as there is more than one system in use. I confess the more I look into it, the less I am sure of how to properly answer your question. It's anecdotal but I have seen many of those spellings as well.
The Wikipedia page about Japanese romanization may provide some info on how those spellings happened. The last link under External Links (on the same page) also seems pretty relevant to the question you're asking.
A thing I would like to suggest is that "ou" may have been used more when it was more difficult to type and display special characters on the computer. Before cellphones were common it wasn't easy to set up a way to type special characters without a lot of hassle. This was my own experience, however, and may not apply to everyone.