To see why, imagine an "alphabet" of only the two letters A and B. Suppose that in our "language", the letter A is used 75% of the time, and the letter B is used 25% of the time. If two texts in this language are laid side by side, then the following pairs can be expected: Pair Probability AA 56.25% BB 6.25% AB 18.75% BA 18.75% Overall, the probability of a "coincidence" is 62.5% (56.25% for AA + 6.25% for BB).
To see why imagine an "alphabet" of only the two letters A and B Suppose that in our "language" the letter A is used 75% of the time and the letter B is used 25% of the time If two texts in this language are laid side by side then the following pairs can be expected Pair Probability AA 5625% BB 625% AB 1875% BA 1875% Overall the probability of a "coincidence" is 625% (5625% for AA + 625% for BB)