Try running this code removing one by one each \pgfplotstablegetelem. You will see that the 2 subfloats will move step by step to the center. Why? Keeping all of them the result is the following. I need to keep the \newcommand in this way, but at the same time the figure centered, how can I do?
\documentclass[a4paper,twoside,12pt]{book}
\usepackage[T1]{fontenc}
\usepackage[utf8x]{inputenc}
\usepackage[english]{babel}
\usepackage{tikz}
\usepackage{pgfplotstable, filecontents}
\usepackage{subfig}
\usepackage{float}
\newcommand{\iso}[1]{
\pgfplotstablegetelem{#1}{T}\of{coefficienti.dat}
\pgfmathsetmacro{\T}{\pgfplotsretval}
\pgfplotstablegetelem{#1}{K}\of{coefficienti.dat}
\pgfmathsetmacro{\K}{\pgfplotsretval}
\pgfplotstablegetelem{#1}{Q}\of{coefficienti.dat}
\pgfmathsetmacro{\Q}{\pgfplotsretval}
\pgfplotstablegetelem{#1}{Kf}\of{coefficienti.dat}
\pgfmathsetmacro{\Kf}{\pgfplotsretval}
\pgfplotstablegetelem{#1}{n}\of{coefficienti.dat}
\pgfmathsetmacro{\n}{\pgfplotsretval}
\pgfplotstablegetelem{#1}{Klf}\of{coefficienti.dat}
\pgfmathsetmacro{\Klf}{\pgfplotsretval}
\pgfplotstablegetelem{#1}{Qlf}\of{coefficienti.dat}
\pgfmathsetmacro{\Qlf}{\pgfplotsretval}
\pgfplotstablegetelem{#1}{m}\of{coefficienti.dat}
\pgfmathsetmacro{\m}{\pgfplotsretval}
\begin{tikzpicture}
\begin{axis}[
width=7cm,
domain=0:1,
xlabel=$Equilibrium$,
ylabel=$Sorbed$,
legend style={draw=none, at={(.9,.3)}}]
\end{axis}
\end{tikzpicture}
}
\begin{document}
\begin{filecontents*}{coefficienti.dat}
T K Q Kf n Klf Qlf m
400 0.0463 32.9 5.78 0.321 .0840 40.7 .675
500 0.154 23.8 6.33 0.281 .0722 86.8 .354
600 0.115 24.6 6.30 0.261 .0257 247 .280
700 1.68 24.2 12.1 0.153 .9760 32.0 .376
\end{filecontents*}
\begin{figure}[H]
\centering
\subfloat[]{\iso{0}}
\subfloat[]{\iso{0}}
\caption{This caption is centered, but not the 2 subfloats}
\end{figure}
\end{document}
