For words that only have 3 letters, there isn't any 4th letter. For instance with the word “add”, there isn't any much more letters afterwards and thus needs to be the word "insert". The word “addict” is while in the BIP39 wordlist however you would have used “addi” for “addict”. https://getsocialselling.com/story4398623/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator