This sentence means "Although I did a good job that cannot be better, I still need to stay humble."

In this circumstance, 再 means "more/again". It means "Although I can do this job better and better over and over again, I still need to stay humble. Because the extent I achieve doesn't really matter, the humility matters." When we use such sentence, actually we want to emphasizes the latter sentence. That is what we really means. 再 here emphasizes a meaning that no matter how strong the degree can be, an opposite circumstance can still happen. For example, 即使你长得再漂亮，我也不会喜欢你。or 即使你再认真，也做不出这道题。etc.

Furthermore, this structure contains an agreement in opinion. It goes like this "If I could finish my task good enough, I would have the right to be arrogant. But now you remind me humility is the most important thing. Even if I refine my job over and over again and make it the best circumstance, I still don't have the right to be arrogant and I need to stay humble."

Hope this helps.