Just as a heads up, in movies there are several different ratios at play and you'll see the two more common ones as 1.85:1 and 23.5:1. The first is often reconfigured as 16:9 for home releases with only the smallest compromises.
Because movies are commonly put out with one of those two ratios, they had to settle on a fixed value for the width and then add black bars to pad the other so they fit. So movies that are at 1.85:1 ratio in cinemas, like Monsters University, use up 1280 pixels wide and 720 pixels tall with little to no padding at the top and bottom when they get a home release on Bluray or HD cable/satellite.
But a 2.35:1 ratio movie in the cinema, like Iron Man 3, can't fit in that same space because it's wider than Monsters University. So on home release it gets 1280 pixels wide and ~540ish pixels in height and another ~180 pixels of black bar letterboxing. The letterboxing is necessary because television only accept the image if it's exactly 1280x720 so the black bars with no useful picture are added to be displayed on television.
When people are encoding stuff for computers, the computer doesn't care about the image being exactly 1280x720 and the black bars are a waste. They take up enconding time, they take up file size, and they add nothing to the experience. By stripping it away, the picture quality improves (more bits per pixel), the file size goes down a bit (don't have to keep encoding a whole lot of black), and you can show the video in a computer window without that extra black space. It's a win-win. So you get stuff that does 1280x528 which is very specifically every bit of visual image from a 1280x720 source without the wasted space of the letterboxing.
So to be totally clear on my position. 1280x528 absolutely
is HD as it's the exact same image as the 1280x720 source with the letterboxing removed. The key factor is that the width is unchanged and it has all of the source image.
To illustrate another way:
1280x720 Source = HD
1280x544 post letterbox removal = Also HD
When they created the terminology, they were focused on televisions which are very inflexible and require specific formats coming into them. So instead of calling it 1280p, they called it 720p for a lot of reasons. They had to deal with differing heights in DVDs where NTSC had 720x480 and PAL had 720x576 so calling DVD 720i wouldn't make much sense because you wouldn't know if it was NTSC or PAL. So they call it by the height that the televisions require.
But computers are totally flexible and don't require those rigid standards. And because the people around here are awesome, they don't force us to have celeb clips with black bars padding them when they aren't necessary. So they get us clips with all the resolution and all the awesome but without the waste of file size and black nothingness.
It totally is HD in every possible sense. If you put the clip on your PC, connect your computer to your television by HDMI, your software will automatically add the padding so the television can display it in a 1280x720 frame because computers are awesome in ways that HDTVs aren't. It's HD. To say it isn't HD is being unproductive because between any given movie they may be a 1.85:1 or a 2.35:1 movie but the 720p clips from their Blurays will be either 1280x720 or 1280x544 and equally as welcome by our community. Unless you want people to be reencoding fine quality clips with padding simply to make the resulting image 1280x720, it's not a productive position to take. I sincerely appreciate that the letterboxing is removed and I get peeved when someone doesn't know or doesn't care to remove them. This is a computer-based message board sharing computer files to other computer users. We have no use for letterboxing so arguing that 1280x528 isn't HD is not productive and makes a distinction that only matters for television and not computers.