A boo-boo in the article has already been pointed out.

Despite my research (and trust me it was extensive) even I was caught providing an incomplete explanation. This is why many folks do poorly on multi-choice (a/k/a multiple-guess) tests. Some answers are sort of right.
The fact is:
MOST fly hook makers use the "one size up convention" for X sizing, as explained in the article.
As with other hook aspects, this is not universal. Actually, it is not even univeral in the same manufacturer and may differ between hook styles, or materials.
Isn't that special?

So...my bad for inferring it was consistent because, like everything else about hooks, there is no consistency.