Coordinate system of camera reading - distance diffrences

[pspyros]

While using reactivision and puredata i get different results when calculating the distance between 2 fiducials that are at a fixed distance.
More clearly: I have 2 fiducials at fixed distance of 10cm placed on my surface. When they are at the center of the surface the distance is been calculated lets say 0.28 units, but when i place the fiducials at the sides of the surface i get 0.38 units, and these prices differ as i move my fiducials at different areas of my surface. This means that coordinates are not being read in a linear way out of the surface of my table.
Why is that happening? How coordinates are being read?
I have tried both square and rectangle calibration and these cameras: Logitechc615 (HD), sony ps3eye, Genius Eye 320SE and a microsoft camera (i don't remember the type) anyway, same results.