For words that have only 3 letters, there isn't any 4th letter. Such as with the word “increase”, there won't be any additional letters Later on and therefore need to be the word "increase". The word “addict” is while in the BIP39 wordlist however, you would've applied “addi” for “addict”. https://wiishlist.com/story19396092/little-known-facts-about-bip39-seed-phrase-list