For words that only have three letters, there's no 4th letter. Such as with the word “increase”, there are no a lot more letters Later on and therefore has to be the word "add". The word “addict” is within the BIP39 wordlist but you would've utilised “addi” for “addict”. A https://royalbookmarking.com/story19021029/top-guidelines-of-bip39-word-list-pdf