In a multi-junction series connected cell the current through the cell is limited by the cell with the least amount of current. They are series connected to allow each junction to optimally use a narrow portion of the solar or other reference spectrum. So current matching gives the maximum current. The efficiency under a given spectrum versus current matching is near the maximum when the currents from each junction are nearly the same. Many devices have the top cell having a few percent less then the other 3 junctions because this gives the maximum energy. Matching the open-circuit voltages might make sense if the devices were in parallel but this would not allow each cell to optimally use a portion of the solar spectrum.