The /a/ in Thai is a central vowel. This is different than English /a/, which is a front vowel. And /o/ is the open back vowel. That is why it falls between

The /a/ in Thai is a central vowel. This is different than English /a/, which is a front vowel. And /o/ is the open back vowel. That is why it falls between English ⟨pad⟩ and ⟨pod⟩. And if you don't speak a language with such a vowel, any particular utterance will probably be mentally mapped to one or the other.