For words that only have 3 letters, there's no 4th letter. For example with the word “insert”, there are no additional letters afterwards and so have to be the word "add". The word “addict” is from the BIP39 wordlist however you might have used “addi” for “addict”. Mnemonic words examples https://throbsocial.com/story20789113/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator