For words that only have three letters, there isn't a 4th letter. For example with the word “increase”, there won't be any much more letters afterwards and therefore must be the word "insert". The word “addict” is inside the BIP39 wordlist but you would've utilised “addi” for “addict”. In this https://bookmarkvids.com/story20108007/not-known-factual-statements-about-bip39-explained