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.

I mean that I’ve heard either pronunciation. And when addressing to English native speakers, the pronunciation may not be connected to local custom...