Equation of 'Spherical model' used to fit an experimental variogram in R

Question

I was wondering what sets of equations are used to fit an experimental variogram when I choose it to be a spherical model. For instance, I used the following parameters of a spherical model:

  model    psill    range
1   Nug 5384.346   0.0000
2   Sph 9042.061 944.9017

here, sill variance, c = 5384.34 + 9042.06 = 14426.41; and range, a = 944.9017. A spherical model is explained by the following equation (book name: Geostatistics for Environmental Scientists):

variogram = c*{(3h/2a)-0.5(h/a)^3} when, h<=a
variogram = c when, h>a

where, h = distance; therefore, if I plug-in the values 'a' and 'c' given a known distance 'h' using the above equation, the coordinate (h, variogram) should fall on the model. Unfortunately, it doesn't do that for different values of h! Is it possible to know what sets of equations are used in the case of different models (spherical, exponential, etc.) in the 'gstat' package?