Hi, ferny!
That *is* a weird situation indeed. Assuming that you are logged into the panel as the same user that owns the directory in question, you *should* own the directory.
The only thing I can think of *could* be dependent upon how "new" the account is, as the Panel processing functions *might* have had problems between the creation of the domain directory and the creation of the user. While I have never seen that exact thing happen, I have seen user creation time vary greatly on occasion, and sometimes stuff you do from the panel takes a while before it works properly.
How long after setting up the domain did you try to do the "one-click" WordPress install? Are you able to log into the domain, as that user, via ssh or ftp?
If a couple of hours has passed, and/or you *can* log in with that user via ftp or ssh, and the WordPress "one-click" is still messing up, I suggest you generate a support ticket, as something may be "messed up" on the Dreamhost back end. It could also be that something is "borked" with the "one-click" installer. Either way, support may be your only help on that. 
--rlparker