From what I can tell both Sim and Pet Autonomy Levels have defined spots that can be used.
High is to the right
Off is to the left
Low/"medium" is in the middle
I usually have it set to the low "medium" setting. The reason I have it in quotations is because that's where it looks like medium would be if we had a little more control over that setting.
I'm not sure why the Sims Wikia is saying that unless it's an older entry. As based on what I am finding in my game it only has 3 settings that is allowed to be used.
I don't think your game is glitched I just think that's just the way the game is meant to be.