The R-value varies with the density of the mix – the more binder, the less insulation. Additional binder increases the density of the material and hence its heat transmission properties. The mix that we typically specify has an R-value of about 2.4 For a 12” wall, yielding more than R-28 for the wall.
There are other arguments that are made for the inherent thermal mass and hygroscopic properties of the material contributing to the effective R-value. To our knowledge, there is no test data to support this claim, and it does not enter into energy calculations.