I can’t reproduce this but I suppose you are experiencing the case when the content is of different length and the content of one page is shorter than the viewport while on the other page it is longer, hence, a scrollbar appears and takes some space on the page with content exceeding the viewport height only, moving the content to the left.
While there is a CSS workaround to make a scrollbar always appear I find it wasted time and energy on a non-issue like this since scrollbars are a user interface element of the OS in which the browser is running. Apple, for example, doesn’t show any scrollbars at all in its latest OS version, if there is a touch device (e. g. trackpad), and if you do the two-finger scroll gesture on the trackpad, the scrollbar is faded in on top of the site. So, for one, visitors on that OS aren’t experiencing any “jitter” at all anyway, and secondly, forcing a scrollbar is useless there, and as I said, is a non-issue to most website visitors. It only bothers you right now because you’re looking closely.