For words that have only three letters, there isn't a 4th letter. For instance with the word “add”, there are no a lot more letters afterwards and thus has to be the word "increase". The word “addict” is from the BIP39 wordlist however, you would've applied “addi” for “addict”. A https://alphabookmarking.com/story18921907/invalid-bip39-phrase-things-to-know-before-you-buy