Off the top of my head I would say because the cheaper stone is a lot darker throughout the diamond and the arrows look out of alignment where as the pricier stone faces up a lot whiter and looks more symmetrical. Just look at the arrows on both and the dark areas which I think means lots of light leakage. Meaning not as bright. I'm sur Neil, JulieN and the others will chime in.
Mostly, JA is a virtual consignment operation. That means that what they're selling actually belongs to someone else and the seller sets the prices. JA is getting a commission for marketing it. They have hundreds of different suppliers and these two are almost certainly from different sources. Why is one discounted? There's a variety of choices but usually it boils down to the seller wanting to land a sale quickly. A 'good' grade anywhere on the report of a high end stone like this makes it a tough thing to sell and one way to make it move faster is to lower the price.