For words that only have 3 letters, there isn't a 4th letter. For example with the word “increase”, there are no much more letters afterwards and so needs to be the word "incorporate". The word “addict” is within the BIP39 wordlist however, you would have utilised “addi” for “addict”. Finding https://www.livebinders.com/b/3631513?tabid=7dd978d9-81c0-05c0-ecba-7d3b4f616e73