I read that during off peak hours, you can often get your transactions confirmed for less than a sat per vbyte. Is that really true? I know the blockchain treats a sat as the minimum quantum of value, how does it work?

And related, it seems the trezor suite does not allow me to select less than a sat per vb for fee either. If the rate really falls cheaper than that sometime, why does Trezor put an artificial floor under the price? What's their incentive here?