The browser is adjusting the viewport size, which you can’t really control. On pages without enough content to force scrolling, the viewport will be about 30 px wider because the scrollbar is gone on the right. Browser plugins or other UI panels such as a bookmark bar, link bar or the browser inspector also change the game on your content widths. This is only a problem on large monitors though, laptops and lower won’t notice the issue in the first place.

Workaround

We recommend leaving things as they are – your visitors are not likely to notice or care where the content ends up in relation to previous pages in this case.

If it is important that all your content lines up systematically and uniformly, scrollbar or not, you can set a minimum height on the #content container to add artificial whitespace  that will force scrolling. This code goes into the Custom CSS box in Theme Options:

#content{min-height: 500px;}