For words that have only three letters, there is not any 4th letter. Such as with the word “incorporate”, there are no additional letters afterwards and therefore have to be the word "add". The word “addict” is while in the BIP39 wordlist but you would have used “addi” for “addict”. https://socialistener.com/story4291934/not-known-factual-statements-about-bip39-explained