Many tires are made with different amounts of different types of rubber. Most tires do have some amount of real rubber in them at different quantities or percentages. The percentages of compounds and because many are made in different layers and vulcanized under different heat and pressure cycles and different amounts of time.
Of course they are not going to be equal. There not designed to be. A 20K mile tire is completely different as a 100K tire. Yes they use different amounts because of cost and availability of the compounds used. There is only so much real rubber produced on a yearly bassos. You can still make a tire out of 100% real rubber, but the cost would be high because there is only so much real rubber produced in the world. They have to use synthetic's to make up for the lack of real rubber that is available vs how many tires are produced. The more cars that need tires is increasing faster then how much real rubber can be produced. Few could afford a tire made of a high real rubber percentage. Making them not cost effective. There has to be a compromise somewhere.