Interesting. I thought they are all produced at the same factory.
If it is true, it could be that Xiom demands slightly different QC specs. This is not impossible, in fact I very much suspect that different seamless balls have different consistency according to brand.
However, I ordered some Xiom balls early on and never noticed any difference in durability (doesn't mean there wasn't one, but it didn't grab my intention). In fact I would not believe it without some carefully documented
data on it. Impressions on stuff like this are easy to come by and easily wrong (which I don't intend with any disrespect to the songdavid98). I have just learned from work and from TT that stuff like this really need quantitative measures (and descriptions of what was measure and how).
By the way, I definitely have the impression that XSF had some quality control issues for awhile there and I have no idea if they have solved them because I ordered a bunch at the beginning and haven't gone through all of them. Of course, I can't document it with numbers either.
In general, seamless balls are the most durable 40+ balls, probably the most durable balls made since the start of the 40 mm era in the early 2000s. Nittaku Premium 40+ balls are average durability, not to different from their cellulloid, and far better than other seamless balls.