Comparator does not compare correctly in differential mode

I do not know why the comparator does not compare correctly. I use AIN6 as Vin+; ANI0 as Vin-. The code is show in below.  The comparator is work in differential mode.

The output of compare is show in below.

yellow line: input signal of comparator. Frequency: 10Hz; High voltgae: 0.5V; low voltage: -0.5V, Symm: 50%.

blue line: output of comparator.

Vin+ of the comparator is connected to the yellow line signal, the Vin- of comparator is connected to GND. Therefore, the duty cycle of blue line should be 50%. But now the duty cycle of 

blue line is not 50%. I do not know why.