[Someone]

In user dialogs, Windows does/used to do that, too. https://learn.microsoft.com/en-us/windows/win32/api/winuser/...:

“The horizontal base unit returned by GetDialogBaseUnits is equal to the average width, in pixels, of the characters in the system font; the vertical base unit is equal to the height, in pixels, of the font.

The system font is used only if the dialog box template fails to specify a font. Most dialog box templates specify a font; as a result, this function is not useful for most dialog boxes.

For a dialog box that does not use the system font, the base units are the average width and height, in pixels, of the characters in the dialog's font. You can use the GetTextMetrics and GetTextExtentPoint32 functions to calculate these values for a selected font. However, by using the MapDialogRect function, you can avoid errors that might result if your calculations differ from those performed by the system.”