Installing a DPDT Slide switches the wrong way round

Hi All.   Point polarity tester, an idea that I found online. Consisting of a plastic. box with a secure lid, two LED’s one red connected to a green wire and a green one connected to a red wire. A third wire just hang loose??? But being the a right partt ? I cannot remember how to use it. When the other year I had installed 4 DPDT Slide switches on my layout, for WIT point control, one point had a polarity problem, I tried out my newly knocked up tester, and it found the problem straight away. It seems that I had installed one of the switches the wrong way round. But, why should this make a difference???  Best wishes. Kevin