Hi @nick ,
FYI with power output figures they never change the supply voltage as all the components are designed to work at the specified voltage.
What does change is the current being supplied.
Power equals voltage times current (P=E x I).
The higher output power adapter regulates its' current output , whilst maintaining the correct voltage output, to match the demands of the laptop at any given point in time e.g high cpu usage fan rpm increases more current required which will be supplied by the charger, when cpu usage decreases and fan throttles back current supply is reduced
The adapter will only supply what the laptop wants at the time, not any more. This includes keeping the battery charged.
For example a higher output power charger can supply a high demand load and keep the battery charged where a lower output power charger may only be able to supply the high demand load and the battery may be only being maintained at its present state of charge or it may even be discharging helping to supply the load. It may depend on how power distribution is configured in the power management of the laptop.