For words that only have 3 letters, there isn't a 4th letter. Such as with the word “insert”, there are no much more letters Later on and thus need to be the word "include". The word “addict” is during the BIP39 wordlist however, you would have applied “addi” for “addict”. https://bookmarkingfeed.com/story18837997/a-review-of-bip39