For words that have only 3 letters, there isn't any 4th letter. Such as with the word “increase”, there aren't any much more letters afterwards and therefore has to be the word "increase". The word “addict” is inside the BIP39 wordlist however , you might have employed “addi” for “addict”. https://socialfactories.com/story4490294/getting-my-bip39-words-to-numbers-to-work