Why Do Medicare, Medicaid And Veterans Affairs Deal With Drug Costs Differently?

Countries sometimes do things differently from other countries or gain reputations for doing certain things well or poorly.

But within a country, within the same federal government, does it make sense to do things differently among departments or programs that are providing essentially the same service?

To that point, does it make any sense that Medicare, Medicaid, the Department of Veterans Affairs and the Department of Defense (whose Tricare health plan costs big bucks) buy pharmaceuticals and negotiate the prices in different ways?