Does 'compute pair/gran/local' change a simulation ?

Hello all,

I am running a shear test simulation and I would like to see some force chains. I use the command
" compute fc all pair/gran/local id pos force"
but because of that my simulation results change.

I have run 2 tests (the log are in the attachments), the only difference is the use of the command. The steps of the simulation are :
-Definition of the box and particles
-Creation of the particles
-Creation of a top mesh stress/servo
-Top mesh is going down to apply normal pressure
-Definition of a bottom grains group
-Bottom grains group shears
-We look for the components of the force applied on the top mesh

The force following the x ('f_wtop[1]') is smaller in the test with compute than in the test without compute.

How is it possible ?

Thanks in advance

Alexandre Sac--Morane