It’s because you’ve only recently become ranked. Generally it’s recommended for new players to leave their first 20 or so games so their MMR can stabilize. To try and explain this better for you;
MMR works fairly conservatively in order to (ideally) weed out smurfs. There’s two MMR numbers - an “Actual” MMR and a hidden “Provisional” MMR.
The first number - your “Actual” MMR, is where you are currently.
The 2nd number - your “Provisional” MMR, is where the game thinks you should be based on your wins and losses against your opponents.
Both numbers change when you win and when you lose, however your provisional MMR changes more drastically - particularly for newer players. As the two numbers get closer, your games are going to start becoming more even, more fair, and more consistent.
This generally takes on average around 25 games, but has been known to take up to 50 or so.
It’s not ideal, obviously. I’m not even sure it’s a good solution. But it’s the way the game works.
Also something to keep in mind is that Unranked MMR is different from Ranked MMR, but both work the same way.
Generally speaking, try to look back at the game and see what players do differently to you. Keep it simple. Do they make more workers than you - do they make them more consistently than you? Do they put down more production buildings than you? Do they spend money better than you do? Do they have more units than you do?
At the end of the day, your ability to spend your money effectively trumps 90% of anything else in the game because 90% of the time, more stuff beats less stuff. It doesn’t matter if you have 1 colossus if your opponent has 200 marines; the 1 colossus will still lose under the combined firepower of the marines, because the colossus is alone and doesn’t have anything else to support it.
This should never happen, though there’s been known to be a bug that pops up occasionally, that occasionally gives people the master league border even if they’re not actually masters. I don’t think that it’s a thing right now, but it was a common problem for players in bronze-gold level to randomly be awarded the masters border.
Additionally, players who’s profile say they were formerly masters players might have gotten it years ago when the game still had separate ladders for each expansion. WoL was basically dead, so it was incredibly easy to get the masters border in WoL, and it would display on your profile as your highest ever league, even if you weren’t really a masters level player.