Quoting from C-hig
Each frame has a “default font” which specifies the default character
size for that frame. This size is meant when retrieving or changing the
size of a frame in terms of columns or lines (see Size Parameters).
It is also used when resizing (see Window Sizes) or splitting (see
Splitting Windows) windows.
Note that this value is frame specific, not buffer specific, so
the buffer-local remapping of the
default face performed by
text-scale-increase does not have any effect on this.
Note also that a buffer may have a range of disparate character widths
on display, in which case, even if the default character size was on a
per buffer basis, a single value would still not (necessarily) be
applicable to all characters.