Despite the similar names and appearance, the P Zero All Season and P Zero All Season Plus are actually very different tires. Pirelli uses the “Plus” designation to indicate their tires that are designed for the replacement market only. That means Pirelli’s tire engineers are free to design a tire that performs exactly how they want it to, and the budget and available technology are the only limiting factors. Pirelli tires without the “Plus” can be selected by vehicle manufacturers for Original Equipment use. 
When a vehicle manufacturer chooses a tire for Original Equipment use, they always have performance targets they want to achieve. Often the tire doesn’t meet those goals “as is,” so the vehicle manufacturer and the tire company work together to “tweak” the tire to meet the desired performance targets. With Pirelli’s naming system, if a customer buys a “Plus” tire, he or she can be confident it was designed to perform how Pirelli’s excellent engineers wanted it to for its position in the marketplace. If a consumer buys a “non Plus” tire, there is a good chance it has been “modified” from the original design to meet a vehicle manufacturer’s target goals, whatever those may be.