Internal reference voltage of Voltage Regulator - what does it depend on?
The relevant section of the datasheet is below: 



The manufacturer guarantees only that the reference will be between 2.275V and 2.325V at 25°C Tj (junction temperature) and in FLOAT mode and with Vin = 10V

It will change some unknown amount with temperature, but typically -3.5mV for every degree C the junction heats- so if the device is running at 50°C Tj it could be 88mV lower. It could be twice or half that different, there is no guarantee. Also it changes with input voltage, so it might be a few more mV different because of that. 

The actual variation is due to manufacturing variations (doping, exactness of resistor ratios and so on) and is quite tightly controlled for an IC (+/-1% of nominal), which is about as good as your external resistor ratios will be if you use 1% tolerance resistors. If you wish to influence the voltages for charging, you can just modify the resistor values (Ra, Rb, Rc) to compensate for unit-to-unit variations in Vref, but compensating for temperature coefficient or line regulation would be more difficult.