Originally Posted by TECH198
Why is 720x576 up to 1920x1080 all part of what we call "HD" ?
720x576 isn't HD. Also if you look at the link you posted to Blu-ray, it says it is SD. 720x576 is the standard def "PAL" resolution (technically PAL doesn't really exist any more. It was an analogue format with 576 visible lines, The 720 isn't always used on TV stations - it can be other values) and since PAL was an analogue format it didn't have a specifc number of pixels across).
1080i and 720p and above are normally what is classed as high definition today (technically they'd also call a 2.40:1 film HD if it was in a 1280x720 format with black bars or probably even a 2.40:1 film in 1280x533 format without black bars even though that's under 720p used lines.).
The Blu-ray standard includes both high definition formats and standard definition formats. On Blu-ray extras can often be in one of the standard definition formats (though there are also Blu-rays with high def extras, normally the more recent films/releases). If the content (eg. extras) were originally SD they can just be copied over. Though there have also been titles that are upscales from SD to HD (ie. encoded on the Blu-ray as 1920x1080 even though the source was originally SD).
But my question is why are these two formats part of the same HD specification
They aren't both part of the HD specification, they're both part of the Blu-ray specification, for the reasons specified above (eg. to also include content that was originally shot in SD).
Also, for the UHD-Blu-ray spec, while they've added UHD formats (eg. 3840x2160 at various frame rates up to 60 fps), they've not included the SD formats or 3D (that was in the Blu-ray format) but they have included 1920x1080 formats (in 2D, but not the interlaced versions).