Ya 720p vs 480i in it's native master form, not the byproduct sent out to your STB though. It has to do with programmers and the packages they sell. They sell a SD pkg that you probably have to take or you cant get the HD. Its the same as wanting that high tier channel in a low tier pack. The program providers dictate what goes where and force you to take the low tier pack just to get the high tier add on's. You just can't buy the high tier stuff by itself except on c band. It's a vicious greedy circle.
Getting back to rez if they didn't butcher the SD feed (left as is on the master) in the first place it can get very close to HD they sell the consumer. Since its butchered so much they have to provide the HD butchered version to get the clarity the SD master provides unbutchered. And of course charge you more.
Its called business as usual, attach everything to the little guys wallet. Corporate Greed in America.