For words that have only 3 letters, there isn't any 4th letter. For instance with the word “increase”, there aren't any more letters Later on and thus needs to be the word "include". The word “addict” is during the BIP39 wordlist however, you would have utilised “addi” for “addict”. This https://thegreatbookmark.com/story18993670/a-review-of-bip39-phrase-list