You know what I think can happen, I've had many sets of the Prielli Cinturatos on several cars, including my wife's. I remember one time a new set I got for my car was not good on wet roads. That tire had always been great in the rain. I think on that iteration Pirelli may have made a little tread change and may have also changed the composition of the tire. I almost wasn't going to get them after that tire, but then i read good reviews and the next set was fine.

I wouldn't doubt Michelin plays around with tread and the compostion of the tire from time to time.