To understand the answer you first need to understand how polarizing works.
I know this is an article about sunglasses but the principals are similar: http://science.howstuffworks.com/innovation/everyday-innovations/sunglass6.htm
Consider the REAL 3D glasses you get at movie theaters. There are two polarized plastic lenses. Each polarized in a seperate direction. If you pop these lenses out and put them over each other will they black out? No! Why? because they are polarized in different directions.
If you put it on an XYZ axis, lenses can be polarized on the X axis, Y axis, or Z axis, or comparison of all three.
Now here is where it comes together. When you take your polarizer in front of your computer screen the screen may be polarizing the light on the X or Y axis, And your polarizer may be polarized on the Z axis.
Hopefully this kind of makes sense, its sort of late here and im trying to think how to explain it so it makes the most sense.
I have two of the same brand, same model polarizer and one blacks out one of my screens but not the other screen, the second polarizer blacks the second screen but not the first. It just depends how the light is polarized.