Why is the TikZ Spy coordinate system different than for the rectangle's?

Question

Problem

I copied some code from here and here and combined the best parts for my work. I am wondering why the coordinates for my rectangle are in percentages of the image while the coordinates of the spy glass are not.

Working Example

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{spy}
\begin{document}

\begin{tikzpicture}[spy using outlines={circle,red,magnification=5,size=1.5cm, connect spies}]
\node [anchor=south west,inner sep=0] (image) at (0,0) {\includegraphics[height=10cm]{tux}};
\begin{scope}[x={(image.south east)},y={(image.north west)}]
    \draw[red,ultra thick,rounded corners] (0.62,0.65) rectangle (0.78,0.75);
    \spy on (4.5,7) in node [] at (3,5);
    \draw[help lines,xstep=.1,ystep=.1] (0,0) grid (1,1);
    \foreach \x in {0,1,...,9} { \node [anchor=north] at (\x/10,0) {0.\x}; }
    \foreach \y in {0,1,...,9} { \node [anchor=east] at (0,\y/10) {0.\y}; }
\end{scope}
\end{tikzpicture}
\end{document}

Second Attempt

In this example, I tried putting the spy using outlines={circle,red,magnification=5,size=1.5cm, connect spies} within the \begin{scope} parameters, but this does not work. The image is zoomed at the wrong location. My idea came from the pgfmanual (68 Spy Library: Magnifying Parts of Pictures p.746 of v.3.0.0), but alas! It did not work.

\documentclass{standalone} 
\usepackage{tikz} 
\usetikzlibrary{spy} 
\begin{document}

\begin{tikzpicture}[]
\node [anchor=south west,inner sep=0] (image) at (0,0) {\includegraphics[height=10cm]{tux}};
\begin{scope}[x={(image.south east)},y={(image.north west)},spy using outlines={circle,red,magnification=5,size=1.5cm, connect spies}]
    \draw[red,ultra thick,rounded corners] (0.62,0.65) rectangle (0.78,0.75);
    \spy on (4.5,7) in node [] at (3,5);
    \draw[help lines,xstep=.1,ystep=.1] (0,0) grid (1,1);
    \foreach \x in {0,1,...,9} { \node [anchor=north] at (\x/10,0) {0.\x}; }
    \foreach \y in {0,1,...,9} { \node [anchor=east] at (0,\y/10) {0.\y}; }
\end{scope}
\end{tikzpicture}
\end{document}

Answer 1

\spy uses the coordinate system in the scope of the spy using outlines option. To use your special coordinate system for spying your picture, you may define named coordinates.

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{spy}
\begin{document}

\begin{tikzpicture}[spy using outlines={circle,red,magnification=3,size=2.5cm, connect spies}]
\node [anchor=south west,inner sep=0] (image)
at (0,0) {\includegraphics[height=10cm]{example-image}};
\begin{scope}[x={(image.south east)},y={(image.north west)}]
    \draw[red,ultra thick,rounded corners] (0.62,0.65) rectangle (0.78,0.75);
    \coordinate (pos spy) at (.2,.2);
    \coordinate (center) at (.5,.5);
    \spy on (center) in node [] at (pos spy);
    \draw[help lines,xstep=.1,ystep=.1] (0,0) grid (1,1);
    \foreach \x in {0,1,...,9} { \node [anchor=north] at (\x/10,0) {0.\x}; }
    \foreach \y in {0,1,...,9} { \node [anchor=east] at (0,\y/10) {0.\y}; }
\end{scope}
\end{tikzpicture}
\end{document}