Found some information on Warm and Natural that I believe will help to explain the batting difference;
http://www.warmcompany.com/wnpage.html
Down in the question and answer section one of the the makers of Warm and Natural provided this answer:
Is warm and natural 100% cotton or is it 80-20?
- "Thank you for your question! Warm & Natural is 87.5% cotton fibers and 12.5% Polypropylene scrim. Warm & Natural is made with 100% USA Grown High grade cotton fibers. The cotton fibers are then needlepunched through a extremely thin stabilizer made of polypropylene, which is a polyester fiber. This stabilizer makes Warm & Natural strong and durable!"
These resins are what may catch fire in the microwave.