For words that only have three letters, there is not any 4th letter. For example with the word “increase”, there isn't any additional letters Later on and so need to be the word "insert". The word “addict” is during the BIP39 wordlist however, you would have utilised “addi” for “addict”. https://bookmarksea.com/story19000915/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting