@andrea, when you watch the video do you view it at "actual size". Sometimes when viewed at something other than the original size this can happen when pixels get "squeezed" together. What you are describing may be on the screen but not on the original video footage. Imagine taking a chess/checkerboard and making it smaller but still having the same number of squares. The extra squares need somewhere to go if the checkerboard is made smaller.
There could also be some other kind of interpolation going on as well.