Larry,
If my memory is correct, and some times it is, the pattern you see is because of the way the ply is made. They "shave" this strips from the log as the log turns. each repeat of the train is the next layer as the piece is shaved from the rotating log. In your mind, roll the strips into a roll and the pattern will line up as the roll gets thicker. The thinner the log gets the close the grain pattern repeats itself.
They say a picture is worth a thousand words – if you understand the words. This video makes a good picture of my explanation.
Larry