Decals are printed by a silkscreen process which is essentially a stenciling technique. There is one screen or stencil for each color ink. The process is done one color at a time. There are pins that hold each screen or stencil in register, but sometimes the screen does not get on the pin properly, or the holes have become worn so the pin does not hold a screen for a given color(s) properly in register.
This is similar to magazines or color pictures in the paper, where sometimes all the pictures on a page have color ghosts because something happened when printing that color. Printing presses, like silkscreening, print one color at a time.