For words that only have 3 letters, there's no 4th letter. For example with the word “incorporate”, there won't be any more letters Later on and therefore need to be the word "add". The word “addict” is while in the BIP39 wordlist but you might have made use of “addi” https://thebookmarklist.com/story18768910/the-2-minute-rule-for-bip39-all-words