Despite what that spec said, each MP3 frame *does* in fact have relative volume level information stored as part of it. Programs like MP3Trim and others *do* take advantage of this information.

I think the reason you didn't see it on that web page is because the information is not stored in the header. It's part of the actual data of the MP3 frame. It's still adjustable by a third-party program without needing to re-encode the frame, but it's not part of the header. Not certain about that, but I think that's the case.