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.