Why does the "w" command display bash's arguments as "ÿ¿"?

Question

The venerable w utility reports who's logged on, what processes they're running, etc.
Why does it print two non-ascii characters, apparently as the (nonexistent) args for bash?

In iTerm2 they render as white question marks on a black background.
Piped to a file and viewed with vi, I see:

15:41  up 5 days,  1:46, 11 users, load averages: 1.67 1.69 1.47
USER     TTY      FROM              LOGIN@  IDLE WHAT
me       console  -                Fri13   5days -
me       s000     -                Fri14   28:15 -bash         ÿ¿    /bin/bash
me       s001     -                Fri15    3:21 -bash         ÿ¿    /bin/bash

When ssh'd into the mac from a Windows PuTTY 0.74 shell, they render as ▒▒ .
The man page for w suggests that it might print a hyphen, but doesn't even hint at Unicode.