For words that only have 3 letters, there isn't a 4th letter. Such as with the word “insert”, there aren't any much more letters Later on and therefore must be the word "incorporate". The word “addict” is inside the BIP39 wordlist however , you would have utilised “addi” for “addict”. https://bookmark-share.com/story19001144/bip39-seed-phrase-mnemonics-generator-an-overview