Lots of people double-batt, especially for competition because it shows the quilting much better. Usually it's an 80/20 blend on the bottom and wool on top.
However, I second the guess that your grandmother was probably using poly batting. You can get different lofts (levels of fluffiness), although a super-high loft batting might a little more difficult to quilt without getting puckers.
I think a looser quilt equals a fluffier, warmer quilt if you use poly or wool. If you think about insulation in houses and down jackets, one of the keys to warmth is having air pockets. Foam insulation has thousands of air pockets in the walls of your house. Down traps warm air in pockets and layers in coats and on a goose. The more densely quilted quilts just don't seem to be as warm to me. Of course, you can then bring weight into it. Heavier quilts always seem warmer!
