Here we have 26 alphabets.
All written on different chits and put in the hat.
We assume the probability of each individual alphabet to be 1/26.
This is just an assumption.
When we really perform this experiment, we may get an alphabet more number of times than actually predetermined.
Such a probability is called a theoritical probability as it is just based on the presumption that all alphabets are equally likely to be drawn from the hat.
Equally likely means, they are likely to be equal but when we really perform the experiment, the ocuurence is different.
Hence we can say it is Theoritical Probability.