I would suspect the reason that some [many?] CF cards work with non-compliant ATA interfaces is due to them not forcing the specification timings. I've definitely seen CF cards that will respond properly to PIO mode 4 timing even though they're configured for the much slower PIO mode 0. This may also explain why only certain brands of CF cards may work on a particular interface and also why physical disks might not. I've experienced hard drives that were picky about being properly configured with timing that matched the selected mode.