ADC difference mode

I use the following code to test the voltage of P0/0 and P0/1，My purpose is to measure the P0/0 voltage first, then measure the P0/1 voltage, and finally measure the differential voltage between P0/0 and P0/1.

The results are as follows:
P0/0 = = = = = = = = P0/1 = = = = = = = = difference mode
0=========== 0 ============ 255
1023======== 0 ============ 1023
0 ========= 1023 =========== 0
1023======= 1023 ========== 511

My understanding is: 1023 is +1.2V, and 0 is -1.2V. 511 is 0V,But when P0/0 and P0/1 are both 0V, why is its difference value not 511, but 256?
When the pressure difference is 0V, shouldn't it be 511?

Test code：

adc_calibrate();
adc_init(GP_ADC_SE,GP_ADC_SIGN,0);
adc_usDelay(20);
adc_enable_channel(ADC_CHANNEL_P00);
adc_value1 = adc_get_sample();
adc_usDelay(1);

adc_calibrate();
adc_init(GP_ADC_SE,GP_ADC_SIGN,0);
adc_usDelay(20);
adc_enable_channel(ADC_CHANNEL_P01);
adc_value2 = adc_get_sample();
adc_usDelay(1);

adc_calibrate();
adc_init(0,0,0);
adc_usDelay(20);
adc_enable_channel(0x0000);
adc_value3 = adc_get_sample();
adc_usDelay(1);