beam scans left to right top to bottom.
in order for the beam to hit a pixel on the far left or right sides, the beam has to be deflected. sometimes a little (as in normal "round" tubes), sometimes allot. (as in shallow and flat tubes.)
in order to isolate the pixel, the electrons have to pass through an aperture grille or a shadow mask just before it hits the phosphors.

this image shows the guns and pixels in a triangle shape...just align them in a straight line "BGR" so the pixels end up RGB. kthx.
now imagine the beam having to hit the phosphors at an extreme angle on the sides... the electrons have to make some pretty knarly turns (90 to 110+ degrees in some cases) in order to be able to hit them without hitting any adjoining pixels. to do this... the phosphor dots and the open sections of the aperture grille are spread a little farther apart. in the middle it's no big deal... the beam goes straight on, no problems.

pixels don't move. only the electrons streaming out of the color guns are susceptible to magnetic fields. which is why the color changes when you get a magnet close to the screen... the electron beams are being moved to hit the wrong phosphor dots.