XY and papersize

["Claus, Matt - eSpeed" <MClaus@espeed.com>]

Hello - I recently discovered Context so let me first of all say a big
thank you to Hans for creating such an amazing piece of software and
making it available.

Now, the question - this works exactly as I expect:

\setupcolors[state=start]
\setuppagenumbering[state=stop]
\setuppapersize [XY][A4]
\setuppaper[nx=3,ny=10,dx=0mm,dy=0mm,topspace=5mm,backspace=5mm]
\setuplayout [location=middle]
\setuparranging [XY]
\showframe
\starttext \dorecurse{15}{test \recurselevel \page} \stoptext

However, when I change A4 to letter the output seems to introduce some
"dy".  I experimented with some other paper sizes including defining my
own and see inconsistent results.  Can anyone point out what I'm doing
incorrectly?

Thank you very much for any assistance,
Matt


\setupcolors[state=start]
\setuppagenumbering[state=stop]
\setuppapersize [XY][letter]
\setuppaper[nx=3,ny=10,dx=0mm,dy=0mm,topspace=5mm,backspace=5mm]
\setuplayout [location=middle]
\setuparranging [XY]
\showframe
\starttext \dorecurse{15}{test \recurselevel \page} \stoptext 


CONFIDENTIAL: This e-mail, including its contents and attachments, if any, are confidential. If you are not the named recipient please notify the sender and immediately delete it. You may not disseminate, distribute, or forward this e-mail message or disclose its contents to anybody else. Copyright and any other intellectual property rights in its contents are the sole property of eSpeed, Inc.
     E-mail transmission cannot be guaranteed to be secure or error-free. The sender therefore does not accept liability for any errors or omissions in the contents of this message which arise as a result of e-mail transmission.  If verification is required please request a hard-copy version.
     Although we routinely screen for viruses, addressees should check this e-mail and any attachments for viruses. We make no representation or warranty as to the absence of viruses in this e-mail or any attachments. Please note that to ensure regulatory compliance and for the protection of our customers and business, we may monitor and read e-mails sent to and from our server(s). 

For further important information, please see http://www.espeed.com/full-disclaimer.html