I took a quick look at your data, and I didn't really see anything too unexpected. It may be that you are surprised by the behavior of the interpolation of the overlay when the underlay dataset has a different grid. There is some user control for this in the "Define Datamode" menu. Then select the Olay resampling and the Stats resampling methods. With NN (nearest neighbor) interpolation, you should get the same values as with the original grid, if the underlay has a finer grid. The other methods will create smoother images.