Relationship between power supply unit Rated Voltage and Li-ion rechargeable battery Maximum Charging Voltage
I could not find that statement in the datasheet you linked. However, take note of this on the input voltage specification:

What this means is that if you want to charge a battery which requires 12.6V, you must provide 13.6V to the unit in order for it to maintain the 12.6V output.
It's a little confusing, because it reads Input (Power Supply Output) but what this section is trying to tell you is that the power management module (PMM) unit's input requirements are dependent on the output voltage desired.
(Put another way, the power supply unit (PSU) must provide 1V more than the battery charging value, so that the PMM can regulate to that voltage.)
This is common for voltage regulators as well. In order to guarantee a stable output voltage, the input voltage must be higher by some amount.