NTSC video was originally 30fps, but when color was introduced, this was changed to 29.97 to make room for the added information. In order for film to be telecine'd to 29.97 fps from 24 fps, it first needs to be slowed down ever so slightly to 23.976, which is "progressive" NTSC. For some reason, this standard (23.976 and 29.97) was kept the same for broadcast HD content (probably to keep compatibility with standard NTSC).

Blu-Ray has the capability of utilizing true 24fps content, but some players don't like it and won't display it properly. Ady encoded ANH and ESB at true 24fps, but it seems that now he's releasing them at 23.976 in order to improve compatibility. A smart move, in my opinion - the difference isn't particularly noticeable anyway.