I have both types of Rambos in medium weight and haven’t noticed a difference. I tend to see it as a gimmick, because you don’t really lose much weight overall. It’s more a redistribution of the bulk. Noting that the fill weight is measured in grams per square meter, the difference in fill weight of the vari-layer is insignificant – probably 3-5 ounces at most.
The area covering the back is probably close to half of the total filled area of the blanket. So, if your blanket has a total of 4 square meters of filled material, then two of those meters will be covering the back, with one meter on each side piece. That means you are getting an additional 100 grams of insulation on the top of the blanket and losing 100 grams from each of the side pieces, for a net reduction of 100 grams (about 3.5 ounces). Even if your total fill is reduced by 150 grams, that’s still only about 5 ounces of total weight.
Obviously this is just an estimation, but I don’t think I’m too far off. I think they found a way to get people to pay more for less! (I bought mine on clearance.)