The premium you pay is based on the amount of work you perform. It only makes sense that a contractor who performs more work should have more injuries and lawsuits than a contractor who performs less work. The insurance companies base the amount of work that is performed on the payroll paid to employees and amounts paid to subcontractors.
If your subs do carry such policies, your risk of injury to the employees of your subs or your risk of lawsuits from their activity is minimized because their insurance policies will pay the bills. As a result, you should not be charged for the activity of your insured subcontractors (with one exception that will be explained under General Liability Premium Determination).