In this image, you can see that everything is working properly. No matter the mode of the comparator, it will always never output if the side input is greater than the back input. 14 is less than 15 so the output is off. For subtract mode, 14-15=-1 which is displayed as 0.

In this image, things start to get weird. Just like the previous example, there is also another strict rule. If the side input is less than back input, then there will be an output, but none of the outputs are on. Although this image shows that if you put a repeater into the back, it works properly. 

Finally, in this image, we can see that when on comparison mode, having the side and the back inputs having the same signal strength, the out put is on because X is not higher than X. In subtract mode, you can see that it should have no output because X-X will always be 0, but it's 15.