Solar panels can supply a limited amount of power, the more current that is sourced from it the lower the output voltage........
This is the basis upon which the circuit works. The presence of the MAX is to charge the nicad batteries as quickly as possible but at the same time maximising the useful energy provided by the solar panel, it does this by
regulating the input voltage as opposed to the output volts....
If you refer to the app notes, you will see the performance curve for the solar cell, all will be apparent then , I hope 8)