For words that have only 3 letters, there isn't a 4th letter. Such as with the word “add”, there isn't any additional letters Later on and therefore should be the word "insert". The word “addict” is while in the BIP39 wordlist however , you would have applied “addi” for “addict”. https://bookmarkick.com/story18838191/facts-about-bip39-seed-phrase-word-list-revealed