I am afraid there is not enough information to make any intelligent comments. It may be a pressure balance issue, an instrument calibration issue or a system affectation. I'd need to see a simplified P&ID and understand how the measurements are made to comment. I'd guess that the issue may be the individual flows are measured at slightly different temperature and/or pressure than the mixture. If you are comparing the outlet to the inlet flows then I'd suspect a leak or absorption or reaction in the bed. The other issue is that if the meters have different accuracies it may all be in that.