The girth is different, and the quantity of fill is different. I would suppose the heavier bag would be more likely to be accurately rated. The lighter one is more likely to be a 30-40 degree bag in actual use.

Campmor's 20 degree down bags are a good budget buy, if you also take into account that the rating is optimistic and figure on more like 25-30 degree comfort. The Campmor brand bag will be around $120-130.

You might also look into quilts. I get true 20 degree warmth with about 20 oz of down quilt. The hat adds about 3-5 oz and I wear midweight base layer and my wool socks. YMMV.