Televisions nowadays are no longer universally sized. As misleading as it may be, all 55" TV's are not the same size. As companies re-design their products, a number of things change, including the dimensions of the TV. Many people are not aware of this fact, and why would they be? If they are all the same diagonally, it seems logical enough that the width and height would be the same, right?
Wrong.
TV's vary in size, no matter the category, for a number of reasons. The bottom line is that you want to make sure you get a screen protector that actually fits the model you have.