I just found out that Nashville Notation only records in major mode?

Example：

The chord progression in C major is Am-C-F-Dm-E

I chose C major in BIAB！

Nashville Notation display：6m - 1 - 4 - 2m - 3

Here comes the question. I am currently selecting Am mode in BIAB

Nashville Notation display：1m - 3b - 6b - 4m - 5

I don't understand why it's 3b instead of 3
Finally, I realized that although I chose Am, I actually chose A major.
Because the scales in A major are Am, Bm, C # m, D, E, F #, G # dim

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So, my question is why not keep the C major and A minor consistent? Just change the numbers.
Why should we change A minor to A major for Nashville Notation display?
Or is this how the Nashville Notation display is designed?

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