It's not a matter of grammar, Ed T. That happens to be an accurate description of the pronunciation of the word in the different contexts of initial letters of the succeeding words -- phonetics, not grammar; usage, not 'rule'. The 'source' you demand lies in one's own eardrums.

I mean, take your own following quote: read it aloud and you will hear "thuh first", "thuh second", "thee ultimate". If you have any other phonetic result, I will give you a nice big juicy red .

≈M≈