Aliens, however, had its alternate cut made back in the early 90s. That's when its "Special Edition" came out on video. It's 20 minutes longer than the plain old version. Since then, the Special Edition kinda became standard. The previous DVD version of Aliens was the Special Edition.
So what you have on DVD now is both the original theatrical version (that you may not have seen), and the Special Edition (that you're used to) via seamless branching. Try choosing the original cut when the disc prompts you, and see if you can spot the 20 minutes of scenes that aren't in it.