I'd say you're spot on. Depending on where you purchased your store bought quilt, it probably (I did say probably) has polyester batting because it's less expensive than other batting...poofier too so doesn't take as much to make it look fuller. And polyester isn't as warm as cotton.
I'm in western Wisconsin and have flannel sheets on my bed with an electric blanket, a store bought comforter (see note below), a fleece blanket, and 40 year old throw on top of that. I love the weight of all of them.
Note: I just started doing quilting last autumn so haven't made a quilt for my bed yet. I found the perfect pattern for my rennaisance-ish bedroom last month and just got the fabric last week. So it IS on my to-do list. I had to get more practice in before I make one for the bed. Sorry, I felt guilty for being on a quilting forum and not having a bed quilt of my own. (Hangs head in shame. lol)