For words that only have three letters, there's no 4th letter. Such as with the word “insert”, there aren't any a lot more letters Later on and thus has to be the word "incorporate". The word “addict” is from the BIP39 wordlist however , you would've used “addi” for “addict”. https://socialicus.com/story4230430/little-known-facts-about-bip39-seed-phrase-list