What is the difference between a skirmisher(duelist) and a Juggernaut(duelist)?

Hey, I would like some help from you guys to understand better these subclasses. As you see I have trouble differenciating them apart from their tankiness:

-Skirmishers are champions that lack safe tools to get on the champion but once they get on one they stick to them and deal extreme sustained damage. They also have windowed defenses and mobility.

-Juggernauts are champions that are tanky and deal extreme sustained damage, they lack safe tools to get on top of a champion (just like the skirmishers) but are tankier.

Now can someone explain to me how Skirmishers like Jax are so different from a Shyvanna, Skarner, Olaf or Udyr (Juggernauts)

I mean shouldn't these last 4 be classified as skirmishers since they have mobility to stick to opponents like Skirmishers: Shyvanna and Udyr (speed boosts)and Olaf and Skarner(speed boosts and slows).

They also have windowed defenses: Shyvanna Dragon form, Olaf W and R, Udyr W and Skarner W.

They are also excellent duelists like Skirmishers.

Thanks for reading this, hopefully someone can explain to me the differences between these Juggernauts and Skirmishers. :)