For words that have only three letters, there's no 4th letter. For example with the word “insert”, there are no a lot more letters afterwards and so have to be the word "insert". The word “addict” is in the BIP39 wordlist however you would have used “addi” for “addict”. Another https://nanobookmarking.com/story19523898/bip39-phrase-generator-can-be-fun-for-anyone