its a new Surface Pro 3. and i'm using the latest version of Muse.
i was just in a Staples store and loaded the site on a Surface Pro 3 there... it loaded the tablet site at first, as it should, then i tried a refresh, and it brought up the phone version.
i closed the window, repeated the process, brought the tablet version, then the phone version on a reload.
so its inconsistent for some reason. its my client's tablet, and he claims not to have adjusted any default settings that might effect that.