The space between two rows seems to differ if the cases environment is used. Have a look at the following example:
\documentclass{minimal}
\begin{document}
\[
\begin{array}{l}
f(x) =%
\begin{dcases}
-1 & x < 0 \\
1 & x > 0
\end{dcases} \\[2em]
g(x) = x \\[2em]
k(x) =%
\begin{dcases}
-1 & x < 0 \\
1 & x > 0
\end{dcases}
\end{array}
\]
\end{document}
I adjust the space between the rows by adding 2em
but it looks different. Does someone has an explanation for it?