Porsche works with manufacturers to test and approve tires that meet Porsches high standards in braking, speed, handling, etc.
If they meet Porsche's standards for tires, they earn an "N" rating that is "stamped" on the tire. Pedro states identical tires, one with and one without, can vary in price by nearly 100%.
More on Porsche's "N" ratings on tires linked below...
For the Nth time Technical Article by Pedro