It may just be that iPhone users are not in as good a position to make purchases. I’m new to these devices, but they are a lot harder to use for such purposes. I’d be interested to hear how others respond, though.
As a non-iphone or ipad user, I would be interested to know if the UI/OS is essentially the same on the two devices and it’s just the size that is different (from a web-browsing perspective).
I also wonder whether a tablet-user is more likely to be on the sofa, a comforable buying environment; whereas an iphone user is more likely to be doing handstands on a log flume, and whatever the site serves them it just isn’t going to make them feel transactional.
I found out about a product (ebook) I wanted to buy the other day, and because I was sitting in the car, I checked it out on the iPhone. I could have purchased it then and there, but for me, it’s like looking through a keyhole, and I decided to wait until I got home and could explore it further on the desktop.
The UI on iPad/iPhone may or may not be the same. It depends at least in part on the CSS, as you can target these devices with different styles … and even with different content, I believe, though I’m not into that.