For words that only have three letters, there isn't a 4th letter. Such as with the word “insert”, there isn't any more letters afterwards and so have to be the word "insert". The word “addict” is during the BIP39 wordlist however, you would have applied “addi” for “addict”. Artwork is https://letsbookmarkit.com/story18817572/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator