discrete time s function dsfunc.c

hello, i'm studying how to write s-function using C language. I'm confused by the demo provided by matlab. In dsfunc.c, the update function is as follow:

    real_T            tempX[2] = {0.0, 0.0};
    real_T            *x       = ssGetRealDiscStates(S);
    InputRealPtrsType uPtrs    = ssGetInputPortRealSignalPtrs(S,0);

    UNUSED_ARG(tid); /* not used in single tasking mode */

    /* xdot=Ax+Bu */
    tempX[0]=A[0][0]*x[0]+A[0][1]*x[1]+B[0][0]*U(0)+B[0][1]*U(1);
    tempX[1]=A[1][0]*x[0]+A[1][1]*x[1]+B[1][0]*U(0)+B[1][1]*U(1);

    x[0]=tempX[0];
    x[1]=tempX[1];

When I modified the last four lines as

    x[0]=A[0][0]*x[0]+A[0][1]*x[1]+B[0][0]*U(0)+B[0][1]*U(1);
    x[1]=A[1][0]*x[0]+A[1][1]*x[1]+B[1][0]*U(0)+B[1][1]*U(1);

The results change greatly, I'm really confused. They are the same in C, why different in s-function? May someone help me, Thank you!