I beg to differ on the above statement, the piston companies rate the compression on their pistons with the following constants.
.038 compressed gasket
.010 in the hole
.030 over bore
So if a person builds their engine to these specs then there is such a thing as 9.5 compression pistons. However, most of time these specs are ignored or not even measured to find out what the builder actually has on the motor.
I guess what I am saying is that the chances are slim that he has 9.5 compression or that anyone has the advertised compression of the piston manufacturer. But it is not because the pistons are advertised incorrectly.