Went to Wally and checked them out today. That appears to be the way they are constructed. I've used them before and never noticed that. That said, I wouldn't have a problem using them. Being a non ecore it has the blue button dome end bypass, so it construction would be different than the Ecore with the combo valve.
It does appear to be some kind of epoxy like material. The ones I looked at it was fairly uniformly spread.