The full scale of AD7687

The url http://www.analog.com/en/content/Tested_Product_Combinations/fca.html is about tested amp/adc driver.

I can't understand:

How are ENOB bits equal 15.5 if ADI recomended the schema for AD7687 which use only the positive scale of output codes because IN- is zero.

I think if ADC is true differential I must use true differential amp.