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.