leave it Rose...leave it...
Yes I agree that at the moment only a small proportion of children would be getting 4a/5 in Y5.
I feel that many many more are capable of this.
I feel that they get lower marks because the setting of maths in particular and to some extent reading is far too low, right from the start of the EYF profile.
I have a great concern for any child who scores lower than this because as an FE teacher of the 14+ age, for over ten years I regularly encounter children who have slipped what seems tiny amounts, who at stages like the Op's many would describe as doing fine or being on target or needing a little help with writing. Playing this forward does not bode well for the academic outcome at secondary, at all.
This is the group who for the past 10 years have been channeled into Btec courses, I have taught over 500 - and yes I would be worried if any of my children were in this position.
I have three children, 3, 5 and 7.