It doesn't make any sense because it's not the original knob. Rotated fully anti-clockwise corresponds to '0' on the scale on the fridge fascia, so just make a mark on the knob next to the fridge zero when it's fully anti-clockwise and when you rotate it wherever the mark points is where you are on the fridge scale.
and the knob only controls the temperature on mains hook up - doesn't make any difference when on 12v when you're driving along. You do know the 12v only works with the engine running don't you?
Also got a Villa 3 (though a little older than yours) - if you dont have the fridge instruction manual you can click on 'Club Stuff' at the top of this page, then click on 'Downloads', then scroll down; your fridge is most probably the RM212 fridge (mine is, and its looks the same as in your pic);
That should explain everything to you .......... (for the record, our fridge came with no knobs on at all!!)