For words that have only 3 letters, there isn't a 4th letter. For instance with the word “insert”, there are no more letters afterwards and therefore has to be the word "insert". The word “addict” is while in the BIP39 wordlist however , you would have used “addi” for “addict”. https://binksites.com/story8756165/the-5-second-trick-for-ledger-seed-phrase-bip39