Why is theoretical probability called theoretical probability?

It is the theory of what might happen, but not actually what happens. In theory, if you spin a coin 100 times, it should come up on heads 50 times, as there is a 1 in 2 chance of you getting heads on each spin. If you actually do spin a coin 100 times, the total of heads is the experimental probability, so what you actually get. That may not be 50. It is likely to be close to 50 though.