Projected area is one of the components of clamp force calculation. Adding a feed system may not add any additional projected area or the additional area my be insignificant so you may not see any significant rise in clamp force.
The reason for the decrease in clamp force could be that the feed system causes shear heat and thus reduced resin viscosity and as a result lower pressure in the part. So while the total pressure may have increased the actual cavity pressure may have gone down. If there was no increase in the projected area, the clamp force would decrease.
The equation is simple but the underlying effects could be complex.
