Why does a copy editor count the number of characters on a line?
73 x 39 x 27 would give an estimate of the total number of characters in your manuscript.  They presumably have some standard estimate for the number of characters that fit on one page in the final format, and they've divided by that number (not shown) to get 25 pages.

I can't think of a good reason to subtract 1 from the number of lines per page (unless one line is obviously a header or something) so I'd wonder if they just miscounted.