For words that have only 3 letters, there isn't any 4th letter. Such as with the word “add”, there isn't any extra letters afterwards and therefore have to be the word "insert". The word “addict” is from the BIP39 wordlist however , you would have utilised “addi” for “addict”. BIP39 https://funbookmarking.com/story18787711/a-review-of-bip39-phrase-list