I am recording Multi-segment Pressure-Volume Data in raw Voltages. Why do the segments not correctly add up to the composite value?

The Millar Ventricaths for use with the MPVS Ultra large animal system, have multiple conductance segments. This allows you to choose the desired number of segments depending on the size of your heart.  Each segment has an output from the MPVS unit which can be recorded in an individual channel. The unit also has a composite output which gives you a combined volume for the segments used.
When recording the data into LabChart it is not possible to sum the raw Voltage values from the segment channels.  This is because the MPVS Ultra provides individual gain and level shifting to convert the produced 0.4-8V signal to an output range of +/- 10V full scale.  Therefore, in order to sum the individual outputs from the MPVS, each volume segment must be calibrated to mS or uL first.
In the attached example, the composite channel was calibrated using the conductance values (70.67mS and 232.4mS) provided in the Volume calibration in the MPVS Ultra Control interface.  The individual volume segments were calibrated using the individual segment values (10.1mS and 33.2mS). The datafile illustrates the correct summation of the calibrated segmental volumes to equal the composite volume.