I find the wording of that bit of the patent rather unsatisfactory. It says "as the voltage dropped.." and not "as the voltage was reduced". Why did the voltage reduce?
The power supply can operate in constant voltage or constant current modes but the data suggests both changed. Which is only possible if the power supply was adjusted or used near its limits (for example in constant voltage mode but with current limiting kicking in.)... or perhaps there was something between the power supply and the cell?