Why do you have acoustic blankets in different weights, but all are listed with the same NRC?

The blankets come in different sizes, this is why the weight is different. But the acoustci material the blankets are made of, is the same this is why the NRC ( Noise Reduction Coefficient) is the same. The NRC is tested in a material sample. The method and the size of the sample are standardized, so any lab test would produce comparable results. In practical applications, the more sound absorbing material you put in a room, the more sound absorption you get, provided that the surface of the material is exposed as much as possible to be able to absorb the sound. For example hanging blankets all around the room with 4-6 inches gap between a reflective surfaced and the blanket is much more effective than having a stuck of folded blankets in a corner.