I'd say it is more likely a combination of the eyeballs behind the optics (glass is quite subjective), and the conditions they tested in.
The ERS line has been gone for a while now, replaced by the DMR2 and DMR2 Pro.
The DMR2 Pro has a little more forgiving eyebox than the XRS2, but obviously the XRS2 has more magnification. The XRS2, as noted previously, has more options for reticles and illumination, although that's hopefully going to change for the DMR2 Pro in the not too distant future.
I run the XRS2s on my match and practice rifles, mainly because I like having that 25-30x magnification on hand just in case I need it. 90% of the time at matches, I am somewhere in the 15-18x range. My Pro is currently a backup/loaner, as the gasser I originally got it for is wearing a thermal.
To my eyes, the glass in both is excellent. The glass in the Vortex HD2s has never really impressed me, but that's probably just due to my particular eyeballs. Personally, I think the DMR2 Pro and XRS2 are damn good deals for what you get for the price. Glass is great, tracking and durability is phenomenal, features are on point, and the zero stop is really underappreciated for simplicity and reliability.
That's my 2 cents, tho. Someone else's will vary, as you can see from the comments here.