I haven't a clue.

It probably supports 576i (which, surprise it or not, is a HD resolution), and even if you don't set it up for that I reckon the picture quality would be fantastic regardless.

Alternatively, it may be the design of the range - the 27" TV for example should be HD as it's a lot bigger than 11", and rather than have to change the design to add HDMI ports for the bigger TVs, Sony might have just stuck in an HDMI port for every model.