> Why does it work,
Well, they USED to have a really good cross-sectional drawing of the Nalge cap seal, but the company has been bought out and the web site changed. I can't find that page any more.
Basically, rather than just have the flat inside of the cap screw down onto the neck, there is also, inside the cap, a tapered flange which seals against the inside of the neck of the bottle. They got some of their fame from this seal: you knew that a Nalge bottle would NOT leak in the lab, while other brands could and did.