1. The discrepancy may come from errors such as invoices were billed to wrong customers....... but I don't agree, I think that error shouldn't affect the total of control account.
2. The reconciliation is actually a reconciliation of 2 systems(accounting system and operation system). In accounting there is only one control account such as Trade Receivable; In Operation, there are thousand of sub accounts. Errors come from downloading process or system code error, etc...
3. Computer-based accounting should not have discrepancy between Control account and Sub-account, if so, there might come from batches not posted.....

I raised this question because I saw this "reconciliation of control account and sub account" often showing in the job description.