Wales’ status is somewhere between Scotland (with its own legal system, school curriculum and NHS) and Cornwall (a part of England which was once a distinct celtic nation). It was conquered by the English rather than its ruling elites being convinced to voluntarily unite, so was governed as if it was essentially a corner of England where some of the locals speak a funny language (which the authorities tried to cane out of them in schools). Now it has a parliament-like assembly, though without the legislative power of the Scottish one, and Welsh is officially used on signage throughout Wales (alongside English).
I will admit that I did not intentionally include Cornwall; keeping the white between the black and blue was just a matter of keeping the two visually distinct (and the old "rule" of tincture). However if it does qualify as a representation of Cornwall too, I'm on board. Call it a happy accident.
The graphic seems to be going by intention. Same reason that the narrow white diagonals are only part of the St Patrick's cross and not part of the St Andrew's one, even though the Scottish flag does also have white there.
I suppose if you continued the logic of your version, you could also count a small diagonal cross in the centre for Northern Ireland too
And a small part of the red would be part of the Welsh dragon, and some of the white for the background of the Welsh flag. Although I think that moves away from the intention.
I like this. I also think another valid composition would be to allow attribution of the same part of the UK flag to multiple countries. This would only really add a few more percent to Northern Ireland, sharing part of the center with England's flag. It would also push the sum of all flags beyond 100%.
But hey, isn't the UK supposed to be greater than the sum of its parts? Poor Wales though.