This is quoting Stephen Spiers from SimpleViewer - I thought you might know how to fix the problem? An example of a clean install on my site here:
Thanks, Rick
"I have now tried this myself and have found a similar scenario.
I installed WordPress v3.1.3, Atahulapa Theme v3.6.7 and WP-SimpleViewer v2.1.3 and created a gallery with 100% width.
The gallery is offset to the right.
However, on activating the default Twenty-Ten theme, the gallery is positioned correctly so it seems to be related to the Atahualpa theme.
When using the Atahualpa theme, the WordPress page displaying the gallery loses all line breaks about half way through the <head> section (view source to see) and validating the page shows that there is a closing </div> tag missing.
Workarounds would be to either use a different theme or to give the gallery an absolute pixel value width rather than a percentage.
Steven Speirs
SimpleViewer Support Team."