For words that only have 3 letters, there isn't a 4th letter. For instance with the word “insert”, there won't be any more letters Later on and therefore has to be the word "insert". The word “addict” is from the BIP39 wordlist however, you would have used “addi” for “addict”. https://worldsocialindex.com/story4527142/phrases-with-bit-an-overview