Obviously the flight paths used in my DC school are somewhat different to the ones being described, as no teacher makes them up, or invents them. It is all done by an external agency, who takes the Year 6 SATs, the CATs, and I am not sure what else, to come up with a "target grade" for each year, and an "aspirational grade" (which is always a grade higher), and which then shows a flight path in a linear way to GCSEs. BUT next to those targets (which they clearly tell us are done by an external agency who has never met your child), they have the teacher's assessment of whether your child is meeting the target, below or above, very much below or very much above (that means the teacher, for each child, has to put a --, -. =. + or ++ in a box for achievement). Next to that box there is another box for effort, which the teacher also has to mark, and then a further box for behaviour (good, satisfactory or cause for concern). On the final reports there is usually a one line comment box as well.
So from this data you can tell if the teacher thinks your child is putting in effort (if the target is being met, but there is low effort, then clearly the teacher thinks the child could do better with more effort). If the target is not being met, but there is lots of effort and good behaviour, clearly the teacher thinks the agency got it wrong, and the target is wrong.
In our case DD has a ridiculously low target for maths. That (in my view) is because nobody picked up at KS2 that she needed extra time, and she didn't finish the SATs maths paper (although still got 105). This was compounded by the fact that the day they sat the maths CAT, she had a dreadful cold, and said she couldn't really do anything as she spent the whole session wiping her nose. She has now been assessed by the SenCo, and extra time allocated, and for her in maths that means in practice at least an extra mark per extra minute.
Her maths teacher not surprisingly put a ++ for attainment, because she is getting one to two levels above the aspirational grade every time since she got the extra time. But, in many ways the target is meaningful, because we have been told that they cannot guarantee extra time once she starts KS4, and if they take the extra time away, I imagine she will go back to scoring several grades below what she is capable of with extra time. The flight path isn't wrong, the teacher isn't wrong, and the data may well give a pretty fair assessment of likelihood of her GCSE grade (both with and without extra time)! But it is not something the teachers make up at all.