The architecture of PAL TVs is optimised for a 50Hz/576 line display, as that is what they will be primarily be used for. They will quite happily switch to 60Hz/480 line mode for NTSC, and will zoom in for non-anamorphic material (losing scanlines off the top and bottom in the process). If I'm correct, this leaves you with just 360 scanlines on screen - on a display where the shadow mask, phosphor dots, etc., are optimised for 576.

When I watch non-anamorphic NTSC material on my TV, I can see noticeable gaps between scanlines.