I'm a bit confused.
In the beginning (chapter 10. page 18) it was mentioned, that the ranks are iron, bronze, silver, gold.
They stated that they would take the bronze rank exam (end of chapter 10 and chapter 11 page 8).
After registering they were wooden rank (chapter 11, page 13), so a new rank.
In this chapter (page 23) the audience talks about them advancing to copper rank, another new rank.
Which ranks do truly exist?
Good catch! I fixed it.
Short answer:
TL shenanigans (as already mentioned)
Long answer:
So in the final page the rank is mentioned twice:
Japanese: ト弱い職業が
銅級冒険者に挑戦するつもりかよ
Romaji: To yowai shokugyō ga
dōkyū bōkensha ni chōsen suru tsumori ka yo
Japanese: ...荷運び人が
銅級冒険者に挑戦するなんて驚きましたか
Romaji: ...Ni hakobi nin ga
dōkyū bōkensha ni chōsen suru nante odorokimashita ka
Literally, 銅 (dō) translates to copper, and 青銅 (seidō) is bronze. As you can see, the Japanese text uses 銅, not 青銅, which is why I opted to use copper by default. While I localized the sentence, I neglected to consider the contextual difference between copper and bronze.
I'm still new to this while learning Japanese and scanlating everything by myself with no proofreaders. Localization is difficult.