If texture sampling is set to repeat, for instance, sampling (x, y) and (x + 1, y) will return the exact same value, whatever x and y. If texture sampling is set to clamp, you'll always get the border values, etc.
When re-constructing the normals, you need to sample a pixel's direct adjacent pixels. And the pixels' coordinates are separated by 1/texture(Width/Height)
I don't know if I'm being clear, but I don't really know how to explain it better in english and without drawing stuff
When you consider the pixel at varTexture0, when you add 1/512, if your texture is 512, then you will sample the neighbour pixel. Try to draw a sample 16x16 pixel texture on a piece of paper, with the upper left pixel being (0, 0) and the bottom right (1, 1), then see what happens when you increase a texture coordinate by 1/16 as opposed to increasing by 1.
Regards.

Find content
Male