OPTISTRUCT : why the results are different if using different number of CPU cores？

Hello, I am doing the frequency response analysis of a chassis structure. I found the result will be different I choose different number of CPU cores. I calculate the same FEM file by adding -nt 2 or -nt 8. The pictrue is the result of displacement response result. I can't attach the FEM file, but I am sure I was using the same FEM file. Is it normal to have different result? thank you.