the reason is as follows, if you were to draw a line from the pushrods to the valves and back what you get is not a rectangle it is a trapezoid (the line running between the valvestems will be longer then the line between the pushrods)
to make up for this the contact point for the valves on the rockers are offset , one to the left, one to the right
trust me, once you get the heads on the engine, the pushrods in the hole and the rockers on the rockershaft it will be obvious
