Why is this table setup producing an overfull warning?
```

\documentclass[9pt,twocolumn]{article}

\setlength{\columnsep}{20.0pt}
\setlength{\columnwidth}{244.0pt}

\usepackage{newtxtext,newtxmath}

\usepackage{graphicx}
\usepackage{siunitx}
\usepackage{booktabs}
\usepackage{tabularray}

\UseTblrLibrary{booktabs, siunitx}

\begin{document}
\begin{table}
    \centering
    \begin{tblr}{
        width=\columnwidth,
        colspec={
        |Q[si={table-format=1.2},l,co=1]|
        Q[si={table-format=5},c,co=1]|
        Q[si={table-format=1.2(3)},c,co=1]|
        Q[si={table-format=2.2},r,co=1]|
        },
        row{1}={guard},
        }
        \toprule
        Step & Cumulative & CI & Maximum \\
        \midrule
        6 & 161 & 8.72(0.18:0.35) & 10.61 \\
        5 & 1462 & 8.75(0.19:0.43) & 10.96 \\
    \bottomrule
    \end{tblr}
\end{table}

\end{document}
```

Edit: also asked at [TeX SE](https://tex.stackexchange.com/questions/736224/overfull-hbox-warning-in-tabularray-with-siunitx-asymmetric-uncertainty)