For words that only have 3 letters, there's no 4th letter. For instance with the word “insert”, there won't be any extra letters Later on and thus has to be the word "insert". The word “addict” is while in the BIP39 wordlist however you might have applied “addi” for “addict”. https://esocialmall.com/story4229825/examine-this-report-on-bip39-mnemonic-phrase-tool