The full-screen start option isn't the same thing as tablet mode - tablet mode optimises the whole experience for touch, with all apps forced to fullscreen (or snapped), various tweaks to the UI to make it more tablet-ish/touch-friendly (wider spacing on the icons on the right of the taskbar, for example), a global back button, no access to the desktop, and running apps hidden on the taskbar (by default, you can re-enable them if you want), plus of course the full-screen start.
The fullscreen start option in the settings is just that, and doesn't enable the other tablet-mode features (so you still have a desktop, apps in overlapping windows, apps on your taskbar, and mouse/kb-optimised UI), but now you have the more win8-ish fullscreen start rather than the smaller menu.