Having compared more what is available, I will make this completely uninformed statement:

The PAL GOUT discs of ANH and ESB were cropped and zoomed when being transferred from the master, removing any advantage PAL might have had in preserving detail.

ROTJ is cropped more at the top but LESS at the bottom allowing it to retain a few more lines of resolution than the NTSC disc. Probably still the same master though.

And one piece of wild speculation: The master used in both versions probably contains MORE detail than is apparent on the DVDs.

I'm starting to wonder if it was intentionally reformatted to letterboxed to reduce detail (and in hopes of a resell in anamorphic later should the inital release go well enough).

The PAL ROTJ would suggest that (since if you used the same NTSC master, there would be no more detail (or lines when formatting) to gain.)

Edit: Although a less conspiracy minded answer might be that some nitwit just overcropped the NTSC ROTJ during tansfer.