„Wrong“ furigana

Hey guys, im writing a paper for a seminar about problems when translating japanese (to german). Now im almost done, but want to add some practical insight so I‘m doing my own translation of a manga I had at home.
An interesting finding was, that the manga (which has furigana) annoted 補欠 (substitute) as ダミー (dummy) which might be easier to understand for readers but I was wondering why they did that. Is the Kanji old, hard to read or not really used anymore? Im also sure they meant 補欠選手 (spare/benched player), since its a sports manga.

Any help would be appreciated 🙏🏻 (its also due this evening so quick replies would be especially lovely😂)

by Koro4n