i send mine to 4 speakers, and it's all in sync.
i've got an iHome iW2, B&W A7, denon surround sound system, and airport express with speakers.
(can also use an aTV hooked to the denon, and that works fine too)
long skinny apartment, so while you can't hear all 4, you can hear 3 at a time.
I'll second auto pilot's guess that the aTV is in sync, but there's something in your speakers/amplifier that is a bit slow.
or on the off chance you've got some distance between the speakers, it might be the delay in the time it takes sound to travel (100 feet will be about 1/10 of a second delay)