Seeing client time drift within domain

I manage a network of 700 POS Win xp machines, located across the state.  These machines are all joined to a domain, with 2 DC's located at my corporate HQ.  

This domain is a child domain of our production, corporate domain.  

I know that by default, client machines sync time with the DC that holds the PDC Emulator role, and I have it set up to do this.

Recently, I have been noticing multiple POS terminals that are seeing some time drift. Some 8, 10, 13 minutes off.  What could be the cause of this?

Thanks in advance!

sb