I have a new XDs with a few hundred rounds through it and have had the same occurrence. It is a technical out of battery condition. The flat firing pin is horizontal in the XDs, not vertical like the Glocks, and this can lead to some confusion. If you examine the far off center strikes carefully, you will see they occur at the top of the primer, which can only mean the barrel is not fully locked. It is locked enough to be in battery and if the gun fires no harm will be done, and you will not be aware of it. If you get a FTF and clear it you will see the off center light strike.
You can demonstrate this to your own satisfaction with a safely cleared and handled gun by manipulating the slide and trigger.
It is clearly a timing issue, but as yet I haven't figured out the solution.
It happens to me usually on first rounds loaded by sling shotting, where I may be inadvertantly retarding the slide, so in normal cycling it may be ammunition related. These little guns are heavily sprung for obvious reasons, and may be ammo sensitive like the Glocks. I'm going to experiment with full power loads and different bullet weights to see what gives.
The XDs is an absolutely outstanding package, and this is just a glitch.