when your source frame rate, and the device frame rate don't match up, you tend to get problems,
check out
this article on wikipedia on Telecine, particularly
this pic to see what i mean.
by going 120, you can divide and get an even number
120/60 = 2 (so each frame of video, gets 2 frames on the display)
120/30 = 4
120/24 = 5
all whole numbers. but...
60/24 = 2.5
you wind up with judder or interlacing trying to make that half frame fit.
the aTV is constantly putting out 60 Fps, i've seem complaints about that, since most movies are in 24