Those Sky HD adverts

[Sciroccology]

I've got a humble BT Vision box, fed via HDMI to an average Toshiba 37" LCD. Normally, tellybox pictures are pretty good, but when those Sky HD adverts come on (sadly they're full of football and stuff) they look AMAZING.

My question is - are they "HD", as such? Is it just increased bandwidth or something? Why does my little telly with a mere 720 resolutions show such an improvement with these ads?

I'm not complaining, just curious.