Hello Reed,
My guess is the same as yours: yes, "ContentChannel" should be
treated the same as "Placement Targeted". (As for why the different
report types return different values--I don't have a good answer for
that.)
I'm going to double-check with some folks from the reports team to
make sure that's the case and will let you know what I hear back.
Cheers,
-Jeff Posnick, AdWords API Team