With Bundler, you have a single copy of each (version of each) library installed, and Bundler decides at runtime which to make visible (which is trivial with Gemfile.lock, since all of the interesting logic is done when creating that). This decision is a global one, so if you depend on two libraries that themselves depend on libfoo, both of those libraries have to use the same libfoo, and there's no difference between having a direct dependency on libfoo and depending on a library that depends on libfoo.

With npm, each library has its own set of dependencies (and those libraries have their own sets of dependencies, and so on) that are entirely separate. Thus, if you depend on two libraries that use libfoo, those two libraries have their own copies of libfoo - and those two copies can be different versions, if needed (and if libfoo depends on libbar, you'll have two copies of that too). This makes the presence of dependencies much less leaky and eliminates most problems with incompatible gem versions at the cost of wasted disk space, (usually trivially) increased memory usage and some potential confusion. It also happens to make npm pretty mind-numbingly simple compared to bundler's dependency resolution logic.