Don't get theoretical probability and experimental probability confused.
Theoretical probability is the probability you'd expect from the outcome of an event based on theoretical principles.
For example, if you roll a fair 6 sided die, theoretically each side is equally likely to land face up. So the theoretical probability of obtaining a 1, 2, 3, 4, 5 or 6 is 1/6.
The experimental probability would occur when actually rolling the die a certain number of times and seeing how many times each side landed face up. If you roll the die 10 times and a 1 lands face up 2 times, the experimental probability is 2/10, or 1/5. This is slightly higher than the theoretical probabiliy of 1/6.