This isn't necessarily a problem, but I wonder about it every time it occurs in my writing.
Horizontal matrices (or single row matrices) have thinner brackets and are differently put out than other matrices. Even Vertical matrices don't have this uniqueness.
Best shown with an example.
\documentclass[10pt,norsk, fleqn]{article}
\usepackage[a4paper, margin=1.2cm,includeheadfoot]{geometry}
\usepackage{amsmath}
\begin{document}
$\begin{aligned}
\vec{x}\ '(t) &= \textbf{A}\vec{x}(t) + \textbf{B}\vec{u}(t)\\
\vec{y}(t) &= \textbf{C}\vec{x}(t) + \textbf{D}\vec{u}(t)
\end{aligned}\ \ \ \to\ \ \
\begin{aligned}
\dot{\textbf{x}} & = \begin{bmatrix}
2&1&1\\3&-2&-2\\1&1&2
\end{bmatrix}\vec{x} +
\begin{bmatrix}
2\\1\\2
\end{bmatrix}\vec{u}\\
\vec{y} & = \begin{bmatrix}0&-1&-1\end{bmatrix}\vec{x}
\end{aligned}$
\end{document}
bmatrix
(and friends) would not be advised. That said, if absolutely perfect alignment is desired, adding (very) small spaces (inmu
units) just before and after the opening bracket and perhaps before the closing bracket should be possible.