Yes, I understand your romanization was just a way to point out phonetic differences.
My question was more regarding aspirated versus unaspirated speech. But your reply tells that’s not what you were intending to discuss, so I’ll drop it.
I think “du” comes from romaji standard that doesn’t focus on sound but which row it is. So for example “t row” is ta ti tu te to and with rendaku it becomes “d row” da di du de do. And the other one focuses more on sound I guess? I’d need to look it up but funnily in my native language (polish) we have distinct “z” and “dz” sounds (yeah, they’re different) and they match japanese pronounciation! For instance polish word “zupa” would be spelled ずぱ and polish word “sadzu” would be spelled さづ (for anyone courious, “zupa” is soup and “sadzu” is declined form (miejscownik) of “sadz” which is a thing for storing live fish)