For words that have only three letters, there isn't a 4th letter. As an example with the word “add”, there are no extra letters Later on and thus need to be the word "add". The word “addict” is from the BIP39 wordlist however , you would've applied “addi” for “addict”. https://advicebookmarks.com/story26446050/bip39-seed-phrase-mnemonics-generator-an-overview