OK – let’s see if I can explain this! Let’s start by ignoring the mobile question for the moment.
WordPress provides the framework and the theme/templates for your website and this includes, of course, the forum page. So – changing your WP theme will often make a difference to the forum display in that it controls that space on the page that the forum utilises.
The SP theme/templates control the display of just the forum within that page space. We can not effect the WP part of the equation.
So – if you use a WordPress ‘mobile’ theme then it may well change the way the forum space is displayed but it will NOT effect the layout of the forum itself. SP will need it’s own ‘mobile’ theme/templates to enable that to happen. And – at the moment – there isn’t one available.
But… we ARE working diligently on one and it is a priority for us. See https://simple-press.com/slowly-going-mobile/ for more details.
I am not really sure if this explains what you want to know. You may ask why there is the option within SP to utilise a separate theme when a mobile agent is being used – when there are no mobile specific themes available. Well – firstly it is preparation for things to come and secondly there have been one or two users who went off to try and make one for themselves who will need that functionality.