[plt-scheme] empty-scene vs. nw:rectangle

Hi all,

Question from one of my students: Why does

   (image=? (empty-scene 200 100) (nw:rectangle 200 100 "outline" "black"))

evaluate to false?  (My best answer, without going digging into the teachpack
code, is that there's a non-visible difference somewhere in the bitmap of the
image.)

Thanks,
jmj