Yeah, it's the scroll bar that you have on every page except links and contact.
The only way to get around this is to either add a scroll bar on these pages using the overflow property in CSS or by adding additional content to these pages so a scroll bar shows up.
The scroll bar is something that is automatic and happens when you overflow the container. On every page except the links and contact page, your content does not overflow the container and therefore doesn't need the scroll bar to show up.
For the time being, you can add an empty div with a defined height on both of those pages so they are consistent. e.g.
I had this exact same thing happen to me when I first started out. Thats why I suggested it might be the scrollbar without seeing your code or page. I remember thinking "whats going on?" when I saw my page shifting over like you did.