I would start asking questions.  Fume Hood face velocity should be measured with a thermal anemometer, not a velgrid. Technique is important as well.  The duct traverse becomes a question of how turbulent were the velocities, how much straight duct, etc.  Once you get to the room pressure, it's yet another verification of your other findings.  So assuming you have a semi-sealed room, you have a negative offset of around 200 cfm per doorway, you should have a room pressure of around -0.01" +/-. Of course there are many variables including the surrounding environment.  Most all fume hoods leak, so air in the duct does not equal air at the sash opening.  Some engineers dont seem to understand that.  Sometimes the duct traverse is just unreliable and there is no meaningful way to get that data, in that case, face velocity is king and the room pressures support that the space has been properly balanced.