As the solution --- being explicit about the paper size --- suggests, it is likely there is a disconnect somewhere regarding the actual paper size ('letterpaper' vs 'a4paper'). The package
geometry usually guesses well, but if it can't figure it out, it defaults to the
dvips driver, which may have different defaults from what you are actually using.
a4paper option helps clear up any confusion, but if you want to explore your system defaults, there are a few options.
An old classic is to do
latex testpage.tex # or pdflatex testpage.tex
and follow the prompts. It will output a document that shows clearly whether your defaults are what you think they are. If I use
a4paper on my computer, which is set up to use
letterpaper by default, it is quite clear that the typeblock is positioned incorrectly.
If you use a TeX Live that includes
tlmgr (i.e., not one installed from your OS's repositories), then it is easy to check and also change what your paper defaults are.
will list the default paper size for all drivers (ConTeXt, dvipdfmx, dvips, pdftex, xdvi). If you don't like what you can see, then you can do
tlmgr paper a4 # or: tlmgr paper letter
This will change the default for all known drivers to
letterpaper). In the unlikely case that you'd like to exercise finer-grained control, you could do something like
tlmgr dvips paper a4
to set a specific driver.