I like how you give subtle hints by using the replacement letters as the most obvious—but erroneous—answer to prevent us from going astray.
Completely unintentional. The computer picks the substitutes. I should check for profanity, or offensive words, I suppose. I remember once the newspaper cryptogram began ABCD EFG HIJ. The letter substitution is arbitrary. A popular form of cryptogram is a “book cypher” where each letter of plaintext is substituted for based on a corresponding letter in a book. For instance if the message was:
ATTACK AT DAWN.
and the book began:
CALL ME ISHMAEL.
A + C = D (1 + 3 = 4)
T + A = U (16 + 1 =17)
....
etc. If the sum is greater than 26, subtract 26.
In the book, “The Good Soldier Schweik”, an Austro-Hungarian General is demonstrating this cypher to his officers and boasts about its impenetrability (it is not) while a young cadet from the academy, sent to the front because of the shortage of officers, casually decrypts the General’s impenetrable message.