Because they just bought it from you at that guaranteed price. They could go and sell it on the market for more/less, but the price you sold it to CB for remains the same.
They do update that price frequently (minutes, maybe seconds) and there is a premium added to the price. I assume they add this premium so they can then go liquidate it for profit on the exchange as needed.
Have you ever bought/sold on CB? It is exactly that, a fixed price. It's the convenience you pay for to use CB. That price updates frequently, but whatever price you execute at is the price you pay/sell for.