The battery voltage may be shown as steady value ( very low voltage variation ~~~ around 50 to 100 millivolts ~~ depending on the internal impedance of the whole battery bank ~~ ), but the MPPT circuit does vary the output voltage of the controller to vary the charging current depending on the power source power availability.
The equivalent is the matching of load impedance to the variable power source impedance.
You may see the input voltage vary wildly which reflects as a variable output current -- that is done by varying the controller output voltage.
Ideally the input voltage should be several volts higher than the battery voltage for easier MPPT work.
For wind mills, the voltage at peak upper power the voltage should be 2 to 4 times the battery voltage, this way, when the wind is low, producing low power, the MPPT control may be able to supply low charging power where others mills without MPPT may not be close to supply charging current.
Same things goes for small hydro systems.
Regards
Nando