If I remember well it might happen because Civil 3D does not position the Tiff file in the correct way. Tiff files (as also World files) should be placed so that the center of the left upper grid pixel is at the desired location. But Civil 3D places the Tiff so that the top left of the pixel is at the given location.
Since Tiff files are a representation of mean elevations of each square, it is not a huge problem. The smaller the grid, the smaller the problem.