When ran Cost rollup after having Fixed OH my cost getting different

Hi Team,

Need your help,

We running into issue with cost rollup, we are following below steps

1. Copying cost to Source Org 319 (Costed item at This Level and Lower Level)
2. Setup Source organization from 319 to 307 (307 is our Target Org)
3. Run Cost Rollup for all the Organization (Getting item cost at Target Org 307 with include both This level and Lower Level)
4. When we ran cost rollup for only target org 307 (Getting item cost at This level only)
5. Creating Fixed OH
6. If we continues step 3, then we ran Cost rollup for target or all Org ( Item cost not showing correct values)

We unable to understand how cost rollup work different? before OH getting item cost correctly after having OH getting item cost differently.