For words that only have three letters, there's no 4th letter. For example with the word “insert”, there aren't any a lot more letters afterwards and therefore must be the word "increase". The word “addict” is while in the BIP39 wordlist however you would've used “addi” for “addict”. In this https://socialskates.com/story20043225/the-invalid-bip39-phrase-diaries