Why are the front and rear slide rails offset on my Craftsman table saw?
The rear rail is shorter because the rear clamp of the fence does not need as much side to side space as the near side of the fence. Here is a picture of a typical table saw fence:



Notice how the near side has the widest part of the assembly.

The measurement scale on the front rail is lined up so that the index arrow on the front side of the fence shows the distance from the saw blade to the side of the fence. The recommended distance from your instructions, using the edge of the table as a reference, is an easy setup. However that may only apply specifically to that model saw with a particular fence and a particular thickness of saw blade. Also note in the picture above how the index mark arrow is offset from the actual fence side.