According to Android documentation (ContactsContract.RawContacts), the DATA_SET column provides another degree of uniqueness.
The data set within the account that this row belongs to. This allows multiple sync adapters for the same account type to distinguish between each others' data. The combination of ACCOUNT_TYPE, ACCOUNT_NAME, and DATA_SET identifies a set of data that is associated with a single sync adapter.
I'm simply not understanding the use case. If there are two accounts with the same type and name, but different DATA_SET values, are they really two separate accounts with their own sets of contacts? Why does this occur? Why not just require different names and eliminate the DATA_SET column?