Put overly tight trousers on someone with a belly and you get Muffin Top. Put vertical scrollbars on some (but not all) pages of a website – you get a similar effect.

Image courtesy of National Cancer Institute
Some browsers hide the vertical scrollbar when a page isn’t longer than the screen. When you move between tall and short pages the page moves position underneath your mouse – links you may have been hovering on, (or avoiding) are now shifted sideways by about 10 pixels. Your eye has to follow – curves follow easier than foldovers.
A simple bit of CSS will trick some browsers to show a scrollbar all of the time.
html {
height:100%;
margin-bottom:1px;
}