Why does the 'a' sound change to 'o' or 'o' change to 'a' with words that start with "w"?


Please consider the following:

[code]

    Warrior is ウォリアー： this vowel sound "a" changes to ”オ"
    Work is ワーク:this vowel sound "o" changes to "ア"
    Chariot is チャリオット:this vowel sound "a" stays "ア"
    Corridor is コリドー:this vowel sound "o" stays "オ"
    As for the other words that don't start with "w", "a" stays "ア", "o" stays"オ".
    Margin is マージン："a" stays "ア"
    Border is ボーダー:"o" stays "オ"....etc.
    
[/code]

But only the words with starting with "w" are opposite. Do you know why?

