Partial work order completion is different compared to when we run costing after all completion

Summary:

we are testing 2 scenarios

1. We created work order of 100 qty and did partial completion of 50 and ran costing and then completed 25, again ran costing and finally completed rest of 25 and again ran costing
2. we created work order for 100 qty and did partial completion of 50 qty and this time we did not run costing and directly did the next completion of 25 qty and then completed rest of 25 qty and after all completion ran costing at end

and noticed that costing for 3 completions are different in both the cases.

1st case the completion cost was different for all 3 completions and for 2nd case completion cost was same for all 3 completions