Generally speaking, more expensive coats will have thinner leather and denser wool, providing the same amount of warmth with less weight. This has to do with where the sheep were raised: sheep raised in colder mountain climates (like Merino sheep) by necessity possess thicker, finer wool for warmth, whereas sheep raised in the plains need less wool and typically have heavier hides. In the case of the B-3 Bomber, it's modeled after the coat used by the airmen of WWII, who flew at high, cold altitudes in unpressurized cabins. The bomber jackets they wore to protect them from the elements came from the plains, so these are rugged, heavy sheepskin coats (not only are these coats tough, they feel tough!) On the flip side, the Highlands II coat provides the same warmth as a heavier B-3 jacket but is a much lighter, thinner coat.