DBA arrearage when short pay

If I have a short check and my DBAs are set to arrear with limits (Q) why do the deductions calculate partial calculations for each rather than all of each until the gross is depleted: Example: Have Dental, Medical and Vision. When I have just 8 hours of pay its does not calculation all of dental, then all of medical and partial on vision. It calculate a partial amount for dental, a partial amount for medical and no vision. What is the basis for the arrearage when set to P, Q, or R