For words that have only 3 letters, there is not any 4th letter. For example with the word “add”, there are no additional letters Later on and therefore has to be the word "insert". The word “addict” is while in the BIP39 wordlist however you would have applied “addi” for https://dftsocial.com/story20565427/bip39-phrase-generator-can-be-fun-for-anyone