Due to the physics of transmitting data over TCP/IP, a hard sync is not possible. Not by anyone. No. Don't argue.
It is a very tough problem, which can only be achieved in highly contrived situations.
The best way to deal with it, is to use well designed gigabit ethernet switches, with good Cat 5e or 6 rated cabling. A good interconnect is worth its weight in gold.
As for how things are streamed, the first player that starts the Xine stream is the master (what we call the Source), and other players slave to it, with the master sending copies to the other devices. No. IP Multicast will not magically make everything sync.
So yes, what you are seeing is a problem, and it is not one that can easily be solved. Many have tried, with varying degrees of success.