For words that have only three letters, there isn't a 4th letter. For example with the word “insert”, there are no more letters Later on and therefore have to be the word "increase". The word “addict” is from the BIP39 wordlist however, you would have used “addi” for “addict”. By https://socialimarketing.com/story5024923/fascination-about-bip39-english-words