Each division has an intended mmr range. MMR is invisible and changes fairly steadily, accelerating if you trend some kind of streak.
Your LP changes are based on where your mmr is relative to the range your division is supposed to be (example plat one may be 1850-1950)
So if you fall or climb fast MMR moves faster so your "league" status tends to lag behind. When you get d5 mmr in plat one you start seeing +25 -15 LP. Works the other way too. This makes earning a new rank slightly more legitimate and combined with demotion protection prevents players from falling out of their new earned tier suddenly (they don't want most players to be afraid they're one game off from a major demotion and not play)
So if you gone up a tier you have to win more games than lose to keep pushing your LP in your favor.
Grades have nothing to do with the ranked system. Only victories count.