For words that only have 3 letters, there isn't any 4th letter. As an example with the word “increase”, there isn't any extra letters Later on and therefore need to be the word "insert". The word “addict” is from the BIP39 wordlist but you might have applied “addi” for “addict”. https://franciscougju442.bravesites.com/entries/general/Opening-the-Power-of-BIP39-An-Extensive-Consider-Word-Lists-and-Secure-Budgets