I agree with Audio.
All electronic components have a "spread". So for a given light input, the resulting current in one photo diode will be different to another.
If you look at a transistor data sheet, parameters such as hFE have a spread, eg. the data sheet I have for the 2N2222 shows that hFE has a spread of 100 to 300 at a collector current of 150 mA. It also varies with temperature and collector current.
The art of electronic design is to obtain a workable result with imprecise components.
Even resistors have a spread, eg. +/- 5%