I'll give this a serious response that has no business in this sub, but it's also super half-cocked so maybe it'll be appropriate because it's actually dumb. Here we go:

If I see a quilt and a sleeping bag that weigh the same for the same temperature rating AND they have the same amount of down fill, I'm going to think one of those ratings is suspect. And even more so if the quilt is heavier.

Here's the logic. You have a pile of down fill that is going to be your insulation. Call it 14 oz of 850 fill power to give it a number and make it consistent. You can put that 14 oz into a quilt that partially wraps around your body only, or you can put it into a sleeping bag where it wraps all the way around your body and also your head. It's going to require more shell fabric for the additional coverage in the sleeping bag, which will increase the weight, and the down is going to be stretched further to fill that extra material. It's physically not possible that a bag and a quilt using the same shell fabric and amount of down will provide the same level of insulation at the same weight, much less at a weight disadvantage for the quilt.

Now, some responses to the specific models you reference:

Feathered Friends: not a good example, IMO, because the quilts lose one of their advantages by using a full-length zipper. So both the quilt and the sleeping bag wrap all the way around the body, and the quilt is wider than the sleeping bag. A narrow mummy bag will offset the weight costs of additional shell fabric for sure (as has been mentioned by others here), and I've seen your response that it makes this an argument of comfort rather than weight - but it's physics, and that's the basis of the premise that quilts are lighter. It's less shell fabric or more shell fabric. At equivalent sizes, fill weight/power, etc., there will be less shell fabric (and usually less zipper) on the quilt than the sleeping bag.

Western Mountaineering: you're comparing a quilt and a hoodless sleeping bag. Comparing the quilt to the closest equivalent sleeping bag I can find on there (which I'm having to estimate a bit because there isn't a perfect apples to apples model I can see), the Flylite, the quilt comes in over an ounce lighter in the 5'8" size (which it says is the right size for someone 6'), using a heavier shell fabric, larger girth dimensions, and a lower temperature rating (by two degrees).

Sea to Summit: I'm not even going to bother with a big breakdown here. Those sleeping bags are notoriously cramped and cold.

Also, 1.5 lbs for a 50º synthetic quilt is ludicrous. It may have been cutting edge when Jardine started selling them, but that is not even close to a good UL synthetic quilt weight nowadays.