Why trapez shape width changes with text?

Question

What I want is to have shapes that are of some width. Apparently trapezium doesn't want to behave and makes shapes larger if there is less text in them. The more text I put in a trapez, the smaller it gets. Can someone explain this and maybe find a solution so that the trapez behaves as a rectangle with respect to sizes?

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes,positioning}
\tikzset{
    trap/.style={
        draw,
        align=center,
        minimum height=4em,
        text width=5em,
        trapezium,
        trapezium left angle=70,
        trapezium right angle=-70,
        },
    rect/.style={
        draw,
        align=center,
        minimum height=4em,
        text width=5em,
        rectangle,
        },
}
\begin{document}
    \begin{tikzpicture}
        \node [trap] (one)  { not5em}; %this has the largest width
        \node [trap,below = of one] (two) {somehow smaller}; %this is smaller
        \node [trap,below = of two] (three) {the smallest one}; %this is even smaller
        \node [rect, right = of one] (one1)  {5em}; %this has the largest width
        \node [rect,below = of one1] (two1) {same width}; %this is smaller
        \node [rect,below = of two1] (three1) {again same width}; %this is even smaller
    \end{tikzpicture}

\end{document}