It's common for a cotton batting to have scrim made of a hydrocarbon, but I'm surprised that it would be as high as 10%. It agree that it's confusing for it to be listed as 100% cotton, but I guess they get away with that because they are considering the product as batting (100% cotton) plus scrim.
Warm & Natural also contains a non-cotton scrim. If you truly need 100% cotton, you could use Warm 100 (has a cotton scrim) or Warm & Plush (no scrim, thicker cotton), or most other cotton batts that do not have scrim. There are also some batts designed for use in the microwave that don't have any poly content.