This is how I understand it.
Say you run a 2x123 set up until the M61 emits 80 lumens. I don't know what that voltage would be, but let's just say it's 4 volts.
Now, when 2 cells are putting out 4 volts, that means each cell is drained until it's at 2 volts, because they're in series.
For 3...