LVDS input output behaviour

I interfaced FPGA(Kintex_7, LVDS_25, Vadj=1.8v)with external board to 
provide inputs(Analog voltage and reference voltage) to LVDS. I adjusted 
frequencies of signal generators  to 1µHz and amplitudes to least 
possible value 10mv.  When both the inputs are 10mv, comparator output 
is zero. I kept analog voltage 10mv and increased reference 
voltage(51mv) till the comparator turn on. In the next step, reference 
voltage is kept same and analog voltage is increased till comparator 
turn on. This process is repeated to max voltage levels.    Please find 
the attached file for the values noted down. I’m unable to relate this 
to theory. Analog voltage is always less than Reference voltage but 
still why the comparator keeps switching? In the beginning its 41mv 
difference but later it will be 100mv, 250mv …why so?. It would be 
helpful if someone explain the LVDS input output behavior as a 
comparator considering those noted values in the file.