AD7767 2's complement output not as described in Data sheet

Hi, 

I am using the AD7767 24 bit ADC. It is measuring a strain gauge load cell. The front end has a programmable gain differential amplifier Renesas ISL28635. The Reference voltage is 3.3 volts.

What I am experiencing is a decrease in signal range from the ADC. When I vary the strain gauge from an unloaded condition, the output is near zero with an offset. As I increase the load the ADC values increase until I reach +Vin of 2.475 and a -Vin of 0.825 The ADC output is 7FFFFF. When I increase the load to get a +Vin of 3.3v and -Vin of 0 volts the ADC output is FFFFFF. Also when I reverse the load where +Vin is 0 volts and -Vin is 3.3 volts I get 000000.

The data sheet shows the input range to get 7FFFFF is +Vin 3.3 volts and -Vin 0 volts. Why is there a discrepancy in the output?

Any thoughts would be appreciated.