Different disc quality results

Can anyone explain why if I run a disc quality test several times (one after the other) with the same drive and media I get different (worse) results each time? I tried more than one drives (ASUS DRW-24B3ST & LITE-ON iHAS122 B) and they perform the same way.

I never repeated the same test, i always run it one, but i guess that the drive and the discs get hotter, and therefor you get more errors.
Give the drive and the disc some time to cool and then run the tests again. if you still have the same results, then you probably have the worst media or optical drive ever made.