Aaaaaaaah! - I’m always glad when I read such statements!
Let me quote Jan70 (and, implicitly, Spath):
“the errors reported by a CATSâ€™ drive (or a PC drive) can not only differ from the ones reported by another consumer drive, but also from the ones reported by the same drive with another firmware, or by the exact same consumer drive at another time. Obviously, severe physical defects will always cause errors, but all errors are not caused by physical defects, and the same physical defect can cause different amounts of errors on different consumer drives. This is why you should never compare PI/PO plots obtained with different PC drives.”
“Correctness is something absurd, which corresponds to a popular idea that each ECC block of a disc contains a precise number of PI/PO errors, and that measurement tools try to find out this absolute value. Letâ€™s repeat it one more time: PI/PO errors are not on a disc. PI/PO errors are the result of a calculation and they depend not only on the disc but also on the drive and on many other physical parameters, thus no PI/PO value is more correct than another. Just like PC drives, CATS will show slightly different results each time you run a test on the same disc, and none of them is wrong : they just correspond to what the error correction block calculated at that particular time.”
Source: CATS vs. Home-made scans