I think you’re asking about the initial aspiration? That ‘hour’ in Ancient Greek is ὥρα hɔ́ːraː, but its plural nominative is ὦραι ɔ̂ːrai, not ὧραι hɔ̂ːrai?
… It’s not true. The plural of hɔ́ːraː is indeed hɔ̂ːrai. The breathing marks are tiny, and extremely vulnerable to typos; that’s the only explanation I can think of.
Particularly when combined with circumflex.