Vertical Blocks Joined Centrally Not Centrally Aligned When Rendered By Vertically Aligned Style

[mcquackers]

Example code:
https://go.dev/play/p/m_4_YwnUyNI

Summary:
I'm trying to align a check box vertically with its header (which can differ based on the data). It seems that when I join the Header and the checkbox vertically, they're fine; the [X] had 3 sectors of whitespace to the left, and 2 the right.

However, when I then style that joined-string to a minwidth of 21 and align it centrally, the checkbox ends up with 10 sectors of whitespace to the left and 8 sectors to the right instead of 9 and 9.