One is the risk to contractor personnel, and the other is the risk to successfully accomplishing the mission. Because contractor personnel are not soldiers, the Army must protect them physically. While contractors can be armed, they can use their arms only for self-defense. They cannot use their arms to perform perimeter defense or even to protect their fellow contractors. This means that using contractors to perform an otherwise military function may not reduce the green-suit force requirement by the same amount.
Soldiers still have to be present to protect the contractors.
On the other hand, contractor presence almost certainly will reduce the soldier presence by some amount. If the contractor fails to perform, there may not be sufficient soldier presence to accomplish the mission successfully. Operational circumstances also affect risk assessment. In relatively benign environments, such as a humanitarian assistance operation, the risk to contractor personnel may be quite low. Accordingly, their presence may be very high in proportion to the soldier presence.