The space and the period have the same width, but the period is almost centered in the allotted width (a bit shifted to the left, see the image below).
\documentclass[11pt,a4paper,twoside,openright]{book}
\begin{document}
\noindent
1. Press the button \texttt{CTRL. OUT} in the controller.\\
2. Press the button \texttt{CTRLXXOUT} in the controller.\\
3. Press the button \texttt{CTRL.\ OUT} in the controller.\\
4. Press the button \texttt{CTRL.\@ OUT} in the controller.\\
5. Press the button \texttt{CTRL\@. OUT} in the controller.\\
6. Press the button \texttt{CTRL\@.\@ OUT} in the controller.\\
7. Press the button \texttt{CTRL.~OUT} in the controller.
\vrule\texttt{.}\vrule
\end{document}

Line 2 shows the widths are the same. After a capital letter, the \spacefactor
of the period is not relevant. The Computer Modern font however, has a nonzero “extra space” component, which is used when the current space factor is 3000 or more, which is the case of line 5 where \@.
first sets the space factor to 1000 with \@
and then the period sets it to 3000. The extra space has a value whose effect is to add a double space. You can realize this with
\texttt{ctrl. out}
as well.
If you really want to reduce the space after the period, you have to do it manually, say with
\texttt{CTRL.\hspace{0.3em}OUT}
Example:
\bigskip
\noindent
\texttt{CTRL. OUT}\\
\texttt{CTRL.\hspace{0.3em}OUT}
