For words that only have three letters, there's no 4th letter. For example with the word “insert”, there won't be any a lot more letters Later on and thus needs to be the word "add". The word “addict” is during the BIP39 wordlist however, you would've employed “addi” for “addict”. https://livebookmarking.com/story18835789/the-2-minute-rule-for-bip39-all-words