1. grunt (grunt-cli) is command line task runner, not frontend library :), so installing it via bower is strange, but possible due to the fact that bower is using npm as base repository :)
2. package.json store all tool dependencies in your project - like bower or grunt

In fronetend development bower should be handling js frontend libraries in your app like jQuery, Angular.js ... . NPM is for node.js extensions/utilities like grunt, karma.

1. grunt (grunt-cli) is command line task runner, not frontend library :), so installing it via bower is strange, but possible due to the fact that bower is using npm as base repository :)
2. package.json store all tool dependencies in your project - like bower or grunt

In frontend development bower should be handling css/js libraries in your app like jQuery, Angular.js, Bootstrap. NPM is for node.js extensions/utilities like grunt, karma devDependencies.

http://blog.nodejitsu.com/package-dependencies-done-right/