Friction loss at high flow will cause a check valve to create a loss of pressure. But at zero flow it would have to be a pretty strong spring in the check valve to cause 5 PSI loss. Could be just a gauge as being in error a couple pounds either way is not out of normal. You are running so close to the max pressure that pump can build anyway, I would just turn up the pressure switch so it doesn't cycle off while the sprinklers are running. Should still go off when the sprinklers shut down.