Unfortunately, the autoscroll capture function does not work as good as it should, I ended up with the same problem as you in various occasions. Usually, repeating the process gets the page correctly. I just tested the page you said, and this seems to be not the case, after various attempts it only gets the top of the page
You can use another screen capture tool, like FastStone Capture. Though it's shareware now, the last free version is
still floating around, and its scroll capture worked better for me in the past (uh, mouser is going to spank me for promoting the competition...)