For words that have only 3 letters, there isn't any 4th letter. For example with the word “insert”, there aren't any extra letters afterwards and thus have to be the word "add". The word “addict” is while in the BIP39 wordlist but you would've employed “addi” for “addict”. In the https://livebackpage.com/story4365060/not-known-factual-statements-about-bip39-explained