back key doesn't back to the previous web page position

[outofin]

In web browsing, if in the middle of a page, I click a hyperlink that opens a new page, and then click the "Back" button in the browser, it backs to the same position I was reading. You know what I'm talking about, right?

However, if I press the "Back" button on the keyboard, the key beside the 4 arrow buttons, it backs to the previous page but is on the top of the page, instead of the middle where I was reading.

How comes? Any setting controlling this?

I have a thinkpad bluetooth mouse that can do back and forward too. It has no such problem.