Because the importer has zero interest in the shooting sports, takes all the leftover production of whatever mismatched calibers/models are not selling abroad, and dumps them on the US market because Americans will buy anything shiny.
The easier way to tell is with it installed in the slide, push from the rear until it's flush with stop plate, the extended FP will also be roughly flush with the breech face.
Assemble the gun without a recoil spring, pull the slide fully to the rear then measure from the front of the slide to the end of the dust cover, that should tell you the length of travel (it has a full length dust cover right?).
I'd bet if you used Federal primers with the original setup all your problems would go away; if you're dead set on using Winchester primers you can either risk the occasional light strike or mess with heavier springs until you get it to function. Are you sure what you had in the box was not the original hammer spring? What is the OD of the coil? bigger than 0.0315"?
It looks to me like they have an excess stock of wonder finish uppers they're trying to clear out. That finish looks great on a blued frame, but it clashes with the hard chrome frames.