[OS X TeX] geometry

[George Gratzer]

As a mathematician, I always liked geometry, but this baffles me.

Following the document class command, I give the command

\usepackage
[paperwidth
=
155mm
,paperheight
=235mm,textwidth=119mm,textheight=195mm,margin=18mm,tmargin=12mm] 
{geometry}

I typeset the article, and I get 205 mm text height.

If I do

\usepackage 
[paperwidth 
= 
155mm 
,paperheight 
=235mm,textwidth=119mm,textheight=195mm,margin=18mm,tmargin=12mm] 
{geometry}
\textwidth=119mm
\textheight=195mm

everything is fine.

I must be using the geometry package incorrectly. Can somebody explain  
to me how to do this correctly?

GG