Very odd. I thought so too, but the 8th Edition manual says pr -n
prints in n columns,
I'm almost certain that -n here means that -2 is 2 columns, -3 columns, etc., and that there is no actual -n switch. In Posix as well as more modern man pages this is spelled -COLUMNS: see
man.cat-v.org.
while the 9th and 10th say it numbers the lines.
I think this may have come from BSD; it is now part of Posix.