Stu,
I was the original poster and the heads on my and my student's banjo are both mylar. Mine is a fiberskin and hers is a frosted skin that comes on a Deering Good Time.
Thanks to all for your answers and theories. I don't know if this'll help me explain it to her, but I'll let her read all this herself and hope she's satisfied. I know that I sure don't need to know any more.
I'm always amazed at the neat responses people receive on this site.