What's up with the PEX on this shower valve?
That is for PEX-A (Wirsbo/ProPEX/Expansion).  Its the expansion type, not the crimp type that is PEX-B, cheaper, and available at big box hardware stores.  PEX-A is slightly better for strength and leaching, and is available from supply houses but costs a bit more.

The way that PEX-A is, it tries to return to form when expanded, so with a special tool it is stretched out with an expansion ring over it, slipped over that fitting, then it shrinks back down in about 10 seconds and holds on.  No crimp rings or anything other than the PEX itself holding the connection together:



While you probably could put PEX-B crimp style on this fitting, its not supported by the manufactures and probably isn't worth the worry of it slipping off someday.  The ProPEX fitting lacks the barbs that the crimp type fitting has.