It can happen - especially on paper.
My DD has just finished Y11, so the proof of the pudding will come in 5 weeks time.
She has very high target grades... 8s in everything, because she has very high CAT scores (and no SATS due to Covid). But, she is also severely dyslexic and has processing issues. She would work at a grade 8 level in class, and then get a 3 in the exams.
Anything with a coursework component, she got 100% for (we have the marks) but she found it very hard to retain information.
Teach her a maths concept one week and as soon as she grasped it, everything would be correct and very high scores, teach her a second maths concept the following week and the same would happen. However if you chucked in a couple of questions on the first concept, not only would they be wrong, but she would question whether she had ever been taught it, despite having high scores only a week earlier. It was a complete nightmare and nobody really knew what to do bar a lot of past papers and a lot of tutoring inside and out of school.
Huge discussions about whether she should sit Foundation or Higher... what do you do with a child who can do the really hard questions at the end of the paper and is utterly stumped by the easier ones at the beginning. I did feel for the poor teacher.
We ended up choosing to reduce down to 6 GCSEs and a BTEC to try and give her the best chance of getting grades in line with her targets. And sat the Higher papers.
I had to have a lot of individual meetings with teachers to actually try and work out what on earth she was actually likely to get this year - and I still would not be able to predict her grades with any real confidence. Even the teachers were pretty lost as to what she is likely to get... we are all hoping for nice surprises on the 21st!
Happily we do not have to try and deal with A levels as she is off to a specialist college next year for her main area of interest.