For words that only have three letters, there isn't a 4th letter. For example with the word “insert”, there are no far more letters Later on and therefore have to be the word "increase". The word “addict” is inside the BIP39 wordlist however , you would've applied “addi” for “addict”. https://active-bookmarks.com/story18811674/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator