Agree that HBO is probably to blame for this, but I don't see this as an aspect ratio problem. It's more like a brain-dead analog conversion problem. Note that there is no vertical blanking interval on DTV as there is for NTSC analog. Similarly for digital TVs such as LCD flat panels, there is no need for overscan to hide data riding on the picture. An SD digital picture is already trimmed to exactly 480 vertical lines, and the extra 45 lines of the blanking interval
should not be present in the video stream. There is nowhere to put them.
I'll say it again to belabor this point. There is literally no place to "hide" a
picture of the closed caption information on a properly-functioning digital TV. Nevertheless, content providers frequently digitize all 525 lines, including the picture of the CC data, and scale that down to 480 lines, and broadcast the whole thing. This should show up as flashing white lines across the top of the screen. Since the CC data is part of the picture, it's difficult for a DTV to detect it and figure out that it should be cropping this errant mess. Some TV set manufacturers, knowing that it's annoying as hell looking at a flashing white line across the top, and knowing that many content providers don't know what they're doing, automatically crop your picture
again.
If you have a DTV that never shows the flashing white line across the top fairly frequently on OTA broadcasts, then you probably have a set that never shows you all 480 lines of your picture! This is sub-SD resolution.