As Sigur points out, the caption is centered w.r.t. the full tikzpicture, not just the plot, but the plot with labels. If you really don't want this, you might mess around with the bounding box.
\documentclass{article}
\usepackage{float}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}
\usepackage{lipsum}
\begin{document}
\lipsum[1]
\begin{figure}[H]
\centering
\begin{tikzpicture}[scale=1.0]
\begin{pgfinterruptboundingbox}
\begin{axis}[ ymin=0, xlabel = variáveis aleatórias, ylabel = frequência]
\addplot [
domain=0:10,
samples=100,
color=red]
{exp(-x)};
\end{axis}
\end{pgfinterruptboundingbox}
\path[use as bounding box] ([yshift=-8mm]current axis.south west) rectangle (current axis.north east);
\end{tikzpicture}
\caption{Some figure}
\end{figure}
\lipsum[2]
\end{document}

Other options include shifting the caption to the left, or, what might be more elegant, to add the same space to the bounding box on the right.
\documentclass{article}
\usepackage{float}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}
\usetikzlibrary{calc}
\usepackage{lipsum}
\begin{document}
\lipsum[1]
\begin{figure}[H]
\centering
\begin{tikzpicture}[scale=1.0]
\begin{axis}[ ymin=0, xlabel = variáveis aleatórias, ylabel = frequência,ylabel style={alias=ylab}]
\addplot [
domain=0:10,
samples=100,
color=red]
{exp(-x)};
\end{axis}
\path let \p1=($(current axis.west)-(ylab.north)$) in (current axis.east) -- ++(\x1,0);
axis.north east);
\end{tikzpicture}
\caption{Some figure}
\end{figure}
\lipsum[2]
\end{document}

This saves you from having to add an yshift
by hand.
e^-x
byexp(-x)
. – user121799 Sep 29 '18 at 15:43x
legend. – Sigur Sep 29 '18 at 15:47