Like Maec said, are prismatic and biweave not an opposite of each other? Such that if you took an a rated module, applied biweave magic, then applied prismatic magic, you'd end up back where you started. An A rated normal one.
So, whilst I get your point, and factoring that the theory doesn't apply down to the exact value I imagine, the result would still be the same.
I also believe that biweave were given a C class because they matched the weight and other things categories - not because they are the middle of the spectrum of normal shields.
Just my opinion though of course.