So first, op.gg's mmr guesser has been terrible this season, no idea what's up with it.
Now then, the actual process. Mmr/lp are part of what's known as an elo system, which ranks players. Originally used for chess, it spread out a lot because of a few benefits I'll cover once I hit the basics. Essentially, the system puts your rank as 2 numbers, 1 public (lp) and one private (mmr). The private one decides who you're matched with (hence, match making rating), while the public one determines your rankings and such. Your mmr is decided by your wins/losses, and your lp gains/losses are decided by the difference between your lp and mmr.
(note: all mmr numbers here are made up, solely for this example)
So, for example let's say you're silver 3 50 lp, which matches with 1300 mmr. If you're at exactly 1500 mmr, you'd gain and lose the exact same amount of lp for wins and losses, and you'd be playing with people who have a silver 3 mmr. Now we'll shift it. You're still silver 3 50 lp, but you're on a loss streak and you're at 1000 mmr (silver 5). You're playing against silver 5 difficulty people even thoigh you're silver 3, so a win will give you less lp and a loss will make you lose more. This is designed to pull you down to silver 5 if you hold a perfect 50/50 win rate at thus mmr. Finally, you're back winning again. You re-hit silver 3, 50 lp but this time your mmr is 1600. Now you're playing against silver 1 level opponents, so your wins will give you more lp, and your losses are more forgiving and you lose less. While you were silver 3 in all these scenarios, your gains, losses, and even match quality vary massively.
So why do it? Ultimately, it's to allow for more dynamic changes in skill. Mmr is a quick changing stat. A quick winstreak can shoot you up quite quick. This rewards players who are playing well, learned a new skill, finally clicked with an old one, or are on a new account/decayed old one and are climbing fast. They quickly reach their desired skill level game wise, and just need to play some more to get their lp up. Similarly, someone who skated through promos (or was carried) will drop down fast in matchmaking when they stop getting lucky/carried. However, you don't want your rankings to be 100% decided by streaks, so you add in the lp system. Lp is slower. You can only gain like, 28ish lp for a win, no matter how high your mmr is. This means that if you are a super streaky player you won't magically rise from s5 to g5 overnight. If you're a tilter, you won't fall like that either. However, if you keep winning over a long period (higher mmr for a lot of wins) you'll still climb fast and can even skip divisions.
Eating while I post this, so it might not be the most cohesive worded thing. Please let me know if I'm making no sense.
Tl,dr: mmr decides who you're matched against, lp is just rank. High mmr = climb fast, low mmr = fall fast. Using the dual system allows for quick climbing and falling to true elo, and keeps streakiness from deciding your whole rank.
