Because the logic goes in the opposite direction.  

If offset = (+1,+1), then for drawing a profile, that means we want to center the profile at (+1,+1) on the image.  We can implement this by shifting the profile up and to the right, then drawing it on the image normally.

For an InterpolatedImage, that means assume the center of the inferred profile is at (+1,+1) on the provided image.  We implement this by creating the InterpolatedImage normally, and then shifting it down and to the left to put the center where we want it.