For words that have only 3 letters, there isn't any 4th letter. Such as with the word “increase”, there isn't any extra letters Later on and therefore need to be the word "add". The word “addict” is from the BIP39 wordlist however you would have made use of “addi” for https://bookmarkport.com/story21433916/facts-about-bip39-seed-phrase-word-list-revealed