One thing that I haven't seen mentioned that is worth pointing out is that the sequence of words is the actual seed input used in the HD wallet calculations - the 128 or 256 bits you start with is just a way to create the string of words, the bits are not the seed phrase in itself.

This also means you can use any random garbage as a seed input for BIP39 wallets - you can, and someone already has, used stuff like "hello" as the seed phrase, and it works fine. Though most wallets will probably complain and/or disallow you to use something that doesn't follow the standard wordlist and checksum format.