Are those crappy verbatims you are using ? I've seen people around with many problems with them....or perhaps a bad batch.
How are you storing your disc ? Are you keeping the surface clean (dust and scratch free?) out of sunlight and extreme temperature changes ?
It is not normal for a disc to deteriorate in one months time unless you are using those cheap brands and the disc quality is really poor....... As to MCCs, should not be a problem. What would worry me is if the increase is evenly spread across the entire disc, particularly the PIF, and above 4. Your scans are within spics for the PI. Afew variations here and there. Try cleaning the disc (inner to outer motion), remove any dust, inspect for scratches and other defects, and try the scan again.....
Run a DVD speed transfer test and look for sudden drops in reading speed. The curve should be smooth and gradually increasing.