Hi lilduckling
I had the same query early on when i bought my first B+W pol filter. My Nikon CPL II does not have such a wavy mottled surface on close examination.
Schneider explained it to be a reflected artifact off their polarizing foils that the human eye sees, and it is not transmitted thru the filter into the image. The exact physics i dunno. But if you pay attention and scrutinize all the B+W pol filters u can find from the shop, they have the same configuration.
But it works like a charm. So dun worry! Start using it!
Ryan