For some time, machines have been used the production of pre-cancelled first day covers. From the sticking of stamps to printing the cancellation, these processes have been automated. That also explains why the stamps are always in the correct position, straight and equally spaced. The cancellation is then printed on the stamps by a machine.
However, machines make mistakes, too. Recently, I have got a first day cover of the Singapore-Indonesia Joint Issue without any cancellation. This is the joint commemorative cover featuring stamps from both Singapore and Indonesia, on an envelope with a white background. Furthermore, the unstamped covers sold at post offices have a yellow background, which means that I did not paste the stamps myself.