Click Cost Calculation button once, then follow menu Go To - Logs - Cost Determination. See the log & do the corrective action or attach the screen shot here.
There is no rule that Planned cost & Actual cost should be same always. That's the reason, SAP has given the concept of variance. Based on the difference in Planned labor time & Actual Labor time or Planned qty of spare parts or Actual consumption of spare parts, Actual cost will differ from Planned cost.
What is the error you are getting in settlement run??
Note: Please give more details to get precise answer.