Preserve scroll position when navigating back/forward in Safari?
It appears as though you're running into the "continuous scrolling" nightmare, which has become popular in recent times.

On all "regular" websites, this is not an issue from Safari 6+ and since at least Lion.

The problem is that some websites load a page, then when you reach the bottom of that page, it loads the next page below the current one. This is meant to be useful, but in the majority of cases it breaks navigation, since the page reloads on navigation.

There is no solution to this mess, other than to disable continuous scrolling, if the webpage you're accessing gives you that option.