The blankets come in different weights because they come in different sizes.
The material is the same this is why the NRC ( noise reduction) is the same.
The NRC is tested for a material sample. The size of the sample for testing
must be standard, so any lab test has the same comparable results.
But in practical applications, the more sound absorbing material you put in
a room the more sound absorption you get. Of course that is provided that
surface of the material is exposed as much as possible to be able to absorb
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.