So I've been looking at an Exped Synmat HL Sleeping Pad and comparing it to the Exped Airmat HL Sleeping Pad. Both are same dimensions 72 x 25.6/16.5 x 2.8 inches.

The Exped Synmat MW weighs 14.6 ounces R value 3.3
The Exped Airmat MW weighs 13.2 ounces R value 1.7

So there's a 1.4 ounce difference and a 1.6 R value difference.

Does this make sense that you can get an additional 1.6 of r value with 1.4 ounces of material

Just comparing the two because one is roughly double the price....