That sounds like what I do somtimes...
Why you sometimes lose against lower rated players

I have finally figured it out ! They often make moves and you try to see some meaningful sense into them and make a meaningful counter move, only to be confrontend by yet another unorthodox move. Before you know what is happening, the pieces are in a mess all over the board and luck, not strategy or tactics, wins the game. Doesn't happen all the time, but happened to me more than once.
That shouldn't be your problem. If you have a good plan against a player that makes unorthadox moves it won't really matter what he does.
I think the real reason why a 1600 would lose against a 1300 is because the 1600 could be kind of cocky, or being over confident and not thinking like they should have on their game.

Just missing one tactic can be the reason. If a player rated 300+ above me doesn't pay attention makes one bad move and loses a knight, bishop, rook or even a queen I will most likely win that game.
Making one bad move which you normally do not make happens, even gm's do make blunders so now and then.

I think it is just a matter of statistics. The quality of moves a chess player plays is variable, so really good players can lose to lower rated players because of the variability in the quality of play. Higher rated players tend to be more consistent, but no one is perfectly consistent.

But it would be unfair as the gain so much points for just one blunder so maybe have a fixed rate for winning and loosing

If you're not at least an IM, you make plenty of mistakes in your games. We all do. Sometimes you make a bad mistake and your weaker opponent happens to see it. Tough luck for you, but there's nothing mysterious about it

Eh, sometimes people don't notice the date on things.
yeah i didn't read the date at all when i was replying to this post lol
I've done that too! A topic pops up and it doesn't even occur to me to check the date. In many cases, the OP quit chess several years ago

I guess the same reason I float around 1200.. when I'm capable of 1950 strength according to the analysis engine. I played a game a few days ago. The analysis said I played 100% Accuracy and so did the guy I was playing. I still won decisively. And the engine rated the play at 1950. I saved the game and shared it. https://m.facebook.com/story.php?story_fbid=pfbid07HgngZpqXJQ4ZDVK7qG7cGbLT6frHHkzn58axNgLbsph3RshvYwwDgGLpq7q4eLSl&id=100088827126151&mibextid=Nif5oz
I have finally figured it out ! They often make moves and you try to see some meaningful sense into them and make a meaningful counter move, only to be confrontend by yet another unorthodox move. Before you know what is happening, the pieces are in a mess all over the board and luck, not strategy or tactics, wins the game. Doesn't happen all the time, but happened to me more than once.