I created a new issue to track this: #42
The "form factors" value at the moment is the value of the PLASMA_PLATFORM environment variable. This variable is set by plasma-mobile to be phone:handset (2 values) and plasma-bigscreen to be mediacenter. This variable isn't set when launching plasma-desktop, but systemsettings currently recognizes it being empty to mean desktop.
I'm not really sure if anything anywhere currently checks for the value "tablet" at the moment, so I don't think there is any behavioural difference.