>>Thanks a lot in advance.
>>
>>UPDATE. I figured this out and visually my results look the same, but UnitTest is still failing.
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>
>I would use string.Format....

Do you mean instead of StringBuilder?

I figured out the missing piece (I just needed to specify the number after the parameter, e.g. {1,11:N0}) but somehow my unit test was still failing although visually to me the output looked exactly the same.

I decided to trust the visual output and switched to a different method now.