HD on a phone?

Google recently added the facility to rent movies by using the Android Market so you can literally stream them to your phone and today I noticed some films seem to have an HD option:


Now the biggest question here I have to ask is... WHY? Obviously HD is a bit of a buzz word lately because it's getting attached to everything, but your phone is ~4 inches big and doesn't even have the TV standard 1080p resolution? Probably most phones are smaller than 4 inches anyway so are you even going to notice the quality of "HD" compared to the normal? Although perhaps the normal version is really low in quality? :) I couldn't find any documentation confirming the actual resolution/bit rate of either...

Rob @ work suggested that perhaps it's so you can watch it on your TV (using DLNA), but I think because it's streaming it straight from the web that's not actually possible, so therefore perhaps it's for the people with HDMI outputs on their phone? Can't be that many of those?

Or is just to make people think "Oh for a little bit extra I'll get a better quality version" ? :S