Vinyl fence is made of essentially the same material as vinyl siding and vinyl windows NOT PVC pipe. The difference being that PVC pipe does use some of the same ingredients but does not have the weather ability and strength found in siding, windows and fence. Vinyl fence is made much like you would bake a cake. You must have the proper mix of the proper ingredients and cook at the proper temperature for the proper amount of time. If any one of these is not correct you will end up with a less than satisfactory product.
Some manufacturers maintain and advertise that mono-extrusion is better.
The ultraviolet inhibitor is the most expensive ingredient in the extrusion formula. Mono-extrusion also requires less technical expertise on the factory floor. Therefore, mono-extrusion will result in a higher cost to the consumer. CO-extrusion demands a higher investment in equipment, tooling, training and overall manufacturing sophistication, but involves significantly lower material costs. In order to make the product even more affordable, many manufacturers use reground material on the inside layer that may be of a different color than the outside layer.