After looking through the library and a bit of confusion I have just realised that it is working correctly.
When the PGA is used the maximum input voltage will be reduced:
1x PGA = 2.048V
2x PGA = 1.024V
4x PGA = 0.512V
8x PGA = 0.256V
If you put a 1.6V input on the pin and set the PGA to be 2 the voltage will be higher than the ADC can read so it will clip the top 0.6V and just show the maximum value it can store which is 1.024V. That is why it is showing 0.5 and 0.25V for 4 and 8 PGA values as well.
When using the PGA settings it may be worth keeping the voltages below the maximum value for the selected gain. It shouldn't damage the ADC as long as the voltage stays below 2.1V but just to be sure it is probably best not to overdrive the inputs.