It depends how you do the selection. The color selection and the fuzzy selection, without anti-aliasing and a zero threshold,will fully select all pixels, so you cut fully-selected pixels and paste fully-selected pixels.
Otherwise, with most selection tools, things are different. Pixels along the edges are partially selected (this partial selection reflects how much of the pixel is inside the selected shape). So when you cut & paste (or paste & delete), the pixels left behind as well as the ones copied are only partially opaque, and since these partial opacities don't add up to full opacity there is a gap.
For instance, this is a blue layer over a white layers. Ctrl-X+Ctrl-V, there is a visible gap:
But the cut is smooth:
One way to fix the problem is to use Select>Sharpen before the cut/paste, so that there are no longer any partially selected pixels. But the cut is jagged:
With a Select>Sharpen there is no gap:
But you are really still looking at something like this:
But the cut is jagged:
So if you used these tools to have smooth edges elsewhere this isn't acceptable, unless you combine two selections, a smooth one on most of the arm and a sharp one on the joint.