The IR file has one record per woman. A variable such as v208 is defined for the woman, and this is the file you should use for that variable.

The BR file has one record per child, whether alive or in the household.

In the BR file, the data from the mother is attached to the child's record. That means the woman's data is repeated for as many children as she has had. Women who have had no children will be missing entirely. Therefore calculating the mean value of v208 in the BR or KR file is incorrect.