My 28-2 has the same situation. In fact, under certain lighting, I can see a slight difference in part of the frame, the cylinder, and the barrel. It is as if the finishing was done at different times by different people. As others have said, it is slight and pretty typical of Highway Patrolman revolvers.
On a side note:
Laketime, if you had a model 28-1 you had best track that gun down and buy it back. A 28-1 is among the rarest S&W revolvers and is worth thousands of dollars!