# rotating package: positioning does not match documentation

I want a sidewaystable to appear with its top against the inner margin of the text body. According to the documentation of the rotating package, setting

\rotFPtop=0pt and
\rotFPbot=0pt plus 1fil


will place a sidewaystable with its top nearest the edge of the page. However, what actually happens is that the center of the table is positioned at the margin, as is illustrated with the following:

\documentclass[twoside]{article}
\usepackage[counterclockwise]{rotating}
\usepackage[a4paper,margin=50mm,showframe]{geometry}

\rotFPtop=0pt
\rotFPbot=0pt plus 1fil
\fboxsep0pt
\def\tab{
\begin{sidewaystable}
\begin{center}
\fbox{\begin{tabular}{c}
line 1\\line 2\\line 3\\line 4\\\hline
line 5\\line 6\\line 7\\line 8\\
\end{tabular}}
\end{center}
\end{sidewaystable}
}
\begin{document}
\tab\newpage\tab
\end{document}


which shows that the top is neither at the edge of the page, nor at the margin (i.e. 50mm from the edge of the page in this example). How can I get it at the margin?

(Edit by Gonzalo Medina) This image showing the problem mentioned was obtained from the code posted (I added showframe for visualization purposes) using TeX Live2012:

-
@Mico I added showframe to the code and uploaded an image of the resulting document showing the problem mentioned. Do you get a different result? –  Gonzalo Medina Sep 6 '12 at 2:00
@WyboDekker I took the liberty of addind the package option showframe to geometry and uploading an image of the resulting document; I hope it's OK. –  Gonzalo Medina Sep 6 '12 at 2:01
Thanks! I should have thought about that myself. This makes very clear that it is not the top, but the center of the table that is aligned. Still don't know how to align the top... –  Wybo Dekker Sep 6 '12 at 9:08