No, it's not a bug.
I've checked your website and both IE and FireFox center your page it the specified page size (1257 pixels)
Here's how it works:
Since we're using CSS absolute positioning (objects always will be displayed on the same location no matter of the browser screen size), the page is always centered in a 'virtual screen'.
This means that if your page is designed for 800x600 (page properties), the center in browser option will create a virtual screen and so will center it in the 800x600 area.
This is not a limitation of BlueVoda, but a side effect of CSS's absolute positioning. You can't have 'real' centering and WYSIWYG at the same time.
Anyway, for the best results set the page width to the minimum size your webpage requires and it will show up fine in all browsers.