rectangular aspect ratio

[gimp-artist]

I downloaded a picture, which is too big. I selected only what I needed by using rectangular tool. I wanted the selected part to be a square. I adjusted the tool option to aspect ratio. Then I clicked down a certain point with the mouse, and pressed shift button on keyboard while dragging my mouse. I had successfully selected a square. I copied the selection, and pasted it in a new image. I pressed "shift+t" for reducing the pasted floating layer.

Then here is my issue. The scale tool doesn't recognize the initial object that needs to be reduced as a square. It is a rectangular with different ratio. I think I copied a perfect square. Then why do I have a rectangular after I pasted it on another image?