siunitx rounding works only if centering

Question

Using the siunitx package

\usepackage{siunitx}

Consider the following two tables:

1.

\begin{tabular}{
  S[table-number-alignment=center, table-figures-decimal=3, table-auto-round]
}
  3.23958398 \\
\end{tabular}

% => 3.234

2.

\begin{tabular}{
  S[table-figures-decimal=3, table-auto-round]
}
  3.23958398 \\
\end{tabular}

% => 3.23958398

In the former, the number gets rounded, but in the latter it does not. Ran under both pdflatex and xelatex. Can anyone explain?

Answer 1

In the second case, you have not set the table-number-alignment key, which will therefore take the default value center-decimal-marker. The later does not reserve space in the table for numbers of a particular size, and so table-auto-round does not apply.

The entire reason that center-decimal-marker is the default is precisely because it does not require any information about the size of the number in order to carry out alignment without any overfull boxes.