Why does npm sometimes use a different version from elsewhere in the dependency tree rather than install a required module?

I'm well aware npm dependencies are tree shaped, and that two different modules can load two different versions of a dependency at the same time.

So when I see an npm error like:

npm WARN unmet dependency /somedir/node_modules/gulp-browserify
/node_modules/browserify/node_modules/module-deps 
requires stream-combiner@'~0.1.0' but will load /somedir/node_modules
/gulp-browserify/node_modules/browserify/node_modules/stream-combiner, 
which is version 0.0.4

Why doesn't npm install the module-deps dependency (stream-combiner@'~0.1.0') underneath module_deps package's package.json?

Or, more generally: why does npm sometimes not install a module where it is needed, using a different version on that module from elsewhere in the dependency tree instead?