Do you have the scores for her 'mock' SATs?
If so, then simply use them as a comparison to her actual SATs results 'Look, you got 5 more in Maths than in your last test because you worked really hard at that, well done'.
Show her that it is all about progress and hard work, not about absolute levels, and show that you value that progress, rather than meeting some particular standard. If what she comes out of Y6 with is a view that she can improve how she does by working hard, and that you value that hard work, then that's a really good thing to take into her secondary education.
There are some myths around SATs - the one that 'they only measure the school' is a particularly prevalent one. Like it or not, your child's individual progress in secondary - in all subjects - is measured from the baseline of her SATs. Because of this, schools set GCSE targets based on SATs, in order to get a good progress score. Yes, they do often also test at the start of Y7, but progress is ALWAYS measured from SATs. If you adopt the same approach - that progress from this point is what matters, not absolute results - then your child, rather than 'always being measured against some absolute standard they may not be able to achieve' can succeed against a sensible measure. And that's something you should celebrate, not rip up.