The confusion comes from the way the USB loader extracts the images from the hard drive.
Instead of filling them with 0xFFs, like Wiiscrubber does, they copy out the last 0x8000 byes that were valid.
This means it looks like a scrambled disc but the H3 tables are all still as invalid. It will compress though. You will also be able to compress them smaller as the discs won't have the update partition in them (which makes the wii fit release of theirs particularly useless as it doesn't have the board drivers in the image then).
You should be able to tell by looking at 0x4fe00 to 0x4fffC in the ISO - scrubbed ones have data there, extracted ones don 't apparently.