When connected and operating, the bridge should be balanced enough to deliver a +/-20mV signal into A+/A-, If it does more, then the +/-20mV times the Gain hits the 0V/5Vrails at +/-0.020*128=+/-2.5V
Since when connected, everything is a 5V, then a 0.0000V difference between A+/A- is delivering a constant raw value to your calibration routine, and you are probably calibrating out-of-spec ADC noise to get your calibration slope
