There are about 2^67 different Enigma machine initial settings. The inverse probability of the appearance of a real seven letter German word (LUEBECK) twice in a random string of similar length to this message is a number that's pretty close to 2^67. So if you decrypted one ciphertext message with all the different incorrect settings you might expect to see one purported plaintext which isn't correct but which has two appearances of LUEBECK, or a similarly misleading occurence. Since there's also one correct plaintext, seeing LUEBECK twice already puts you at roughly 50/50 that it's the real message versus the most convincing wrong plaintext (if you had no prior knowledge of what the settings might be). The additional presence of even a few of the other recognizable German words (or common abbreviations such as triple letters and the shortened names for the numbers) makes it overwhelmingly likely that this is the correct plaintext. LUEBECK + LUEBECK + STUETZPUNKT in one message make the chance that it's not the real message of the order of winning a jackpot in state lottery two weeks running, even if the rest of the message was gibberish. In practice, much shorter pieces of plaintext than the double LUEBECK (like the presence of a single triple U, one spelled-out number, or highly abbreviated weather info) were used to validate guessed settings with a high degree of confidence.