a2bur:

I thought Blu-ray was all the same and was the system that decided how to display it .

Just like DVD, Blu-Ray can support a number of resolutions and progressive or interlaced image. You player/processor/display can still up- or downscale them if needed.

For example, most "Full HD" AVCHD camcorders produce 1440x1080i content instead of the full 1920x1080. AVCHD discs produced by them are still fully conforming to the Blu-Ray spec.