How do Bower and NPM couple together?

Bower and NPM do work differently.

NPM is very powerful and great at what it does. However, you won't find many client-side packages in there; most of them are there because they work in Node too. (For example, Underscore.JS and the JADE template engine.)

That's why you have Bower, which has the majority of the client-side packages. It has many jQuery plugins, templating engines, CSS frameworks, etc. Don't expect to find such packages in NPM.