I played a lot when at school. I had glandular fever when at university and my concentration was not the same afterwards.
I started playing 10 minute chess on chess.com about 7 years ago - after 40 years of not playing much.
I am very variable - 1600 is my ceiling - but I can drop down to 1280 at worst.
It is partly because I often play when I can't sleep to the point where I can't see the board - but my ability to see the moves also just comes and goes .
I know when I am playing well (by my standards) - there are just so many moves available - but this vision evapourates - chess becomes hard again instantly.
I am bipolar - and chess helps me see how I function in life generally.
In your case you clearly do better when you make fewer mistakes - this will probably be true for all of us.
The better the opponent the less room they will give you to find decent moves - so you may find it more difficult to avoid inaccuracies.
I thought 1350 was a ceiling - until it wasn't. Then 1570 seemed to be the limit - until it was wasn't.
If I limit the number of games I play, and can avoid playing when very tired - I do better.
But I can't - and I enjoy playing knowing that when my grade falls I can probably get it back up again.
Understanding your limits helps performance - but chess is a game, and it is OK to go beyond your limits - perhaps minimizing how low your grade goes is part of playing better overall.
Am I understand correctly that I mostly play a bit better than my opponents? I checked my stats and my average accuracy is 84% when I won and 75.4 when I lost. I am struggling at 1500-1600 rating range, so I was surprised that the graph above shows that I play generally better than my opponents.