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
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.