It\'s not by the luck of draw
Whether you get an RFE or not someone has to look into your case before deciding on the case.
When they look into your case and see any discrepancy they want to clarify it by sending "REQUEST FOR EVIDENCE". As you said in case of employment letter may be for "Re-Verify"(If the case is too old, they might want to see if the person is dead or alive).
In any case if you do not get an RFE it means, the examiner is satisfied with the evidence you have supported for the case and good to be adjudicated. Else RFE.