For words that have only 3 letters, there's no 4th letter. For example with the word “insert”, there aren't any more letters Later on and thus must be the word "insert". The word “addict” is from the BIP39 wordlist however, you would have applied “addi” for “addict”. Yet another advantage https://socialbuzzfeed.com/story4333915/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-seed-phrase-mnemonics-generator