Misaligning table header and caption

Question

for some reasons my table header and caption are being pushed to the right, but the rest of the table is in the centre - any idea what's going on here?

\documentclass[a4paper, 12pt, doublespacing, nolistspacing, chapterinoneline, parskip, liststotoc]{article}

\usepackage{graphicx}
\usepackage{graphics}
\usepackage{rotating}
\usepackage{mathtools}
\usepackage{natbib}
\usepackage{adjustbox}
\usepackage{tabularx}
\usepackage{amsmath}
\usepackage{wrapfig}
\usepackage{tabu}
\usepackage{booktabs} % \toprule
\usepackage{subcaption} % subcaption
\usepackage{siunitx} % S column type (align by decimal)
\sisetup{parse-numbers=false} % allow parentheses in number column

\usepackage[utf8]{inputenc} 
\usepackage{fullpage}
\usepackage[british]{babel} 
\usepackage{booktabs}
\usepackage{mathpazo} % font
\usepackage{pdflscape}
\usepackage{cmbright}
\usepackage[T1]{fontenc}
\usepackage{caption}
\usepackage{array}
\usepackage{multirow}
\usepackage{longtable,booktabs,threeparttablex,threeparttable}
\usepackage{siunitx}

\title{RAE}
\author{U1502015}
\date{\today}

\setlength{\parskip}{1em}
\setlength{\parindent}{0em} 

\newcommand{\rowgroup}[1]{\hspace{-1em}#1}

\begin{document}

\begin{longtable}[c]{lc}
\centering
\small
\caption{Estimates and standard errors of probit regression of binary EWEB on independent variables}\\
\hline
{} & \textbf{Binary EWEB}\\
\hline
\endfirsthead

\caption{Estimates and standard errors of probit regression of binary EWEB on independent variables - continued}\\
\hline
& \textbf{Binary EWEB}\\
\hline
\endhead

\hline    
\multicolumn{2}{r}{(\emph{Continued on next page...})}
\endfoot

\hline
\multicolumn{2}{l}{*** p<0.01, ** p<0.05, * p<0.1} \\
\endlastfoot

    \rowgroup{\textbf{Income (Linear)}} & \textbf{-0.250**} \\
          & (0.111) \\
    \rowgroup{\textbf{Income (Quadratic)} & \textbf{0.029**}} \\
          & (0.012) \\
    \rowgroup{\textbf{Income (Cubic)}} & \textbf{-0.001**} \\
          & (0.000) \\
    \rowgroup{\textbf{Relative Wealth}} & 0.126 \\
          & (0.079) \\
    \rowgroup{\textbf{Work-Life Balance (Linear)}} & 0.368 \\
          & (0.290) \\
    \rowgroup{\textbf{Work-Life Balance (Quadratic)}} & -0.041 \\
          & (0.033) \\
    \rowgroup{\textbf{Social Capital (Linear)}} & \textbf{1.333*} \\
          & (0.788) \\
    \rowgroup{\textbf{Social Capital (Quadratic)}} & \textbf{-0.258*} \\
          & (0.154) \\
    \rowgroup{\textbf{Injunctive Norm}} & -0.041 \\
          & (0.208) \\
    \rowgroup{\textbf{Descriptive Norm}} & \textbf{1.527***} \\
          & (0.543) \\
    \rowgroup{\textbf{Personal Norm}} & \textbf{0.998***} \\
          & (0.379) \\
    \rowgroup{\textbf{Descriptive Norm \# Personal Norm}} & \textbf{-1.569***} \\
          & (0.566) \\
          &  \\
    \rowgroup{\textbf{Age (Linear)}} & -0.027 \\
          & (0.043) \\
    \rowgroup{\textbf{Age (Quadratic)}} & 0.000 \\
          & (0.000) \\
    \rowgroup{\textbf{Female}} & -0.107 \\
          & (0.148) \\
    \rowgroup{\textbf{Heath}} & 0.256*** \\
          & (0.045) \\
    \rowgroup{\textbf{Non-White}} & -0.244 \\
          & (0.202) \\
    \rowgroup{\textbf{Highest Qualification}} &  \\ \\
    \textbf{GCSE level or equivalent} & -0.558 \\
          & (0.490) \\
    \textbf{A-levels or equivalent} & -0.414 \\
          & (0.467) \\
    \textbf{Undergraduate degree} & -0.318 \\
          & (0.462) \\
    \textbf{Masters degree} & -0.391 \\
          & (0.596) \\
    \textbf{PhD} & -0.010 \\
          & (0.574) \\
    \rowgroup{\textbf{Personality Traits}} &  \\ \\
    \textbf{Extraversion} & 0.071 \\
          & (0.054) \\
    \textbf{Agreeable} & -0.027 \\
          & (0.068) \\
    \textbf{Conscientious} & 0.009 \\
          & (0.089) \\
    \textbf{Neuroticism} & -0.169*** \\
          & (0.046) \\
    \textbf{Openness} & 0.025 \\
          & (0.081) \\
    \textbf{Optimistic} & 0.236*** \\
          & (0.065) \\
    \textbf{Adaptive} & 0.116** \\
          & (0.057) \\
  \label{tab:probit}%
\end{longtable}%
\end{document}