It's Yumiduka because it's Yumiduka.
It sounds like Yumizuka to us(and probably to most Japanese people), but du is the vocalized form of "tsu"(or tu, in kunreishiki). Just apply the same sound transformation from ta -> da and to -> do for tsu->dsu, and you'll get something like dzu, which mostly sounds like zu. If you are to type this into your windows IME, and you will find that only Yumiduka gets you the correct kanji, while Yumizuka gets you something like "hot and cold water?"
Hepburn romanization generally lumps the du and zu sounds together and just spells it as zu, and generally in official spellings and whatnot, they use Hepburn because it looks better, so why they chose to leave Satsuki's last name like this, I have no idea.
This kana is not that uncommon though. A common word you hear in melty blood is something like "tsuzukeru", or let's continue. That zu there is actually the same pronunciation as the du in Yumiduka, so you can type tsuzuku into your IME all day and not find the kanji for it, because it's looking for tsuduku.