I was using the Gantry template for a three column layout but a few of the pages needed a wider content space, so we decided to hide the right column (sidebar-b) on those pages. However when we did so, sidebar-a moved to the right side.
The solution can be fond here:
http://www.rockettheme.com/forum/index.php?f=394&t=175596&rb_v=viewtopic#p867304
Essentially, this highlighted something about Gantry template configuration that I had not understood until now; it is not only the configuration slider for the detault number of columns that has an impact on your layout but the configuration you set for two columns will kick in if the layout only has two columns.