That's true, there is an amount of inventory that's always in place. The problem is that when you take one out, you have to replace it. What do you replace it with? You replace with what you paid for it that day. So that answers the question why, even though there's inventory in place, you still get the current price on the current day.
In terms of why Katrina translates immediately, it is because it's such a liquid market and so transparent. If I'm a wholesaler and my choice is to sell to market A or B, and market A just went through a hurricane and needs product and is willing to pay for it, why would I sell to market B at a lower price? It's simply the way the price translates through the marketplace.