According to Xerox, the font "Courier is a 10 point font that will print at exactly 12 characters per inch." Given a default page with 5.5 width, 65 characters per row.
One can quickly verify that 12 pt Courier, and many other 12 pt founts, fit 46 or 47 rows per default page with 9 inch height.
Consider a character width a column. For monospaced 12 pt fonts, it is common to have 60 characters per line. Given 46 rows, the ratio of columns to rows, or height to width in character spacing, is 18:11.
Yet nowhere is this ratio, or any like it, suggested for the total space a monospace or any glyph uses. That is because of the "72 pts per inch" clause.
If there are 72 pts per inch, there are 648 pts in 9 inches, or 14 pts for each of 46 rows.
Similarly, there are 396 pts in a 5.5 width document page. If 60 12 pt characters fit, each is ... 6.6 pts wide! Further, the ratio of 14 to 6.6 is 2.12. Yikes!
Variability between different fonts, etc., could account for some difference. 6.6 to 12 is beyond that.
Can this be explained?
There are two things to understand about font point sizes. First, it only specifies the height of the font, not the width. This should be obvious when you consider that a non-monospaced font will vary in width for each character. Second is that the point size only specifies the height of the character cell, not the row spacing - the relation between the two is left entirely up to the font designer, and can generally be overridden in software. For that matter the size of the character itself within the character cell is also left up to the font designer and can vary significantly between two fonts with the same point size.
For example, in my copy of Microsoft Word the default page is 8.5x11 with 1 inch margins, leaving a printable space of 6.5x9 inches or 468x648 points. I've set my paragraph to single line spacing with no before/after padding. If I use a Courier New 12 point font, each character should be 7.2x12 points, implying 65x54 characters per page. The actual capacity is 64x47 characters. This implies a width closer to 7.3 points, and a row height that is more than 12 points, approximately 13.7 points.
If I set the paragraph line spacing to exactly 12 points, I get 54 lines as expected.
If I switch to Consolas, another monospaced font, the capacity changes to 70x46. The designer of Consolas decided that it could be narrower but taller than Courier, for an identical point size.