I think the 2 circles are sewn together with the batting and that makes the bottom half. The 2 half circles are overlapped and used as the top piece...that way you can tuck the tortillas inside and it still holds in the warmth. Kinda like you do on the back side of a pillow where you can pull the pillowform out and wash the pillow cover...the back side of it is in the shape of a square, but it's two overlapping pieces so the pillow stays secure inside, but allows for it also to be pulled out.
Im sure i didn't explain that real well.