Slope in the viscosity vs shear acceleration graph from non-equilibrium MD

GROMACS version: 2020.1
GROMACS modification: No

In finding out the viscosity of a system with non-equilibrium MD using GROMACS, an acceleration is given to the system due to which, the velocity profile changes. This change in energy is used to estimate viscosity as shown by the equations in this link.
For different molecular systems at the same T&P conditions and similar densities but with different acceleration amplitudes, the viscosity decreases with a different slope, as the acceleration amplitude increases. As the viscosity at steady state is obtained by extrapolating these values to 0 acceleration amplitude, this slope is important in changing the values significantly. Therefore, I would like to know the property that this slope signifies.