This is going to be a long and possibly silly explanation why the rating system does not have to behave like a box on a spring, basically, it boils down to lack of inertia:
A box on a spring will gain speed as it reaches it's intended point (the point the box on the spring would be stable), then overshoot it because he still has speed and can't get rid of it. Then he will be pulled down again and gain speed yet again, but he will cross his destination with a bit less speed due to friction. This will repeat until friction took all it's speed away, because there is no other way for the box to lose it's speed.
Imagine, the box attached to a spring wants to be at 150m, but is at 50m. The string will pull the box towards 150m. The moment the box is at 150m, the spring stops pulling, but the box still has speed, so the box will shoot upwards, to say 200m (friction reducing its speed). Then, the spring will push the box downwards towards 150m again, it has speed and overshoots and this will repeat until the box is steady at 150m.
I'm going to use lp to describe your visible rating including league division etc.
The system however, does not have a limited amount of friction. The system can just pull your lp with an arbitrary speed to your mmr and then stop you on the spot with an arbitrary amount of friction. The system however, limits its speed relative to the distance between your lp and your mmr.
Imagine, your lp is 50 and your mmr is 150. The system therefore thinks you belong at 150 and starts pulling you. Assume you do belong to 150 and you manage a neat 1:1 win/loss ratio. The moment your lp is at 150, the system stops pulling. However, since lp has no speed because it's not a box, it will instantly even out at 150.
There is one way the system dampens you though.
Imagine, you have 100 lp, 100 mmr and 100 mmr is your skill level. A huge win streak will significantly increase your mmr to 150 but your lp only to 120. Then the games after, you will probably go on a loss streak because you play against better players, your mmr drops down to 100 again, your lp drops to 105 because it was lower than your mmr for most of the loss streak. Now you start consistently winning and becoming better, your mmr will slowly grow to 150, but your lp will be following below this, 130.
Your mmr is much more sensitive to win/loss streaks than your lp and lp always follow behind it, dampened. This makes lp more accurate because it's less prone to random win/loss streaks but still accurately represents actual increases in skill.