KP26 Fixed Price & Variable Price - Logic for Fixed price updaion in CK11N

Dear Experts,

I have maintained Fixed and Variable price in KP26 for an activity against one cost center.

The secondery cost elements for such activities I have maintained in OKTZ.

When I am doing Standard cost estimate it showing some difference in cost for the activity.

If I maintained only variable price, the estimate for the activity cost is correctly calculating.

If I maintained only Fixed price, the estimate for the activity cost is differ as compared to kp26 calculation.

If I maintained both the prices say Fixed and vairable price, the estimate for the activity cost is differ as compared to kp26 calculation same as fixed cost case.

I am expecting the following:

1, What is the logic behind fixed cost calculation.

2,Please suggest which cost fixed / variable is suggestable to maintain in kp26 . Here Our activity is machine.

Jyoshna