For words that have only 3 letters, there isn't a 4th letter. As an example with the word “increase”, there are no much more letters Later on and so needs to be the word "incorporate". The word “addict” is in the BIP39 wordlist however you would've used “addi” for “addict”. https://bookmarkvids.com/story20125239/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator