NPM releases have a version of OpenFL that is already compiled. This makes OpenFL’s sources external to your project, which makes OpenFL compatible (and available) on a CDN, and is good for multiple embeds. You have a few minor things you can do that aren’t available in the Haxelib releases, such as
var stage = new Stage (550, 400);
var sprite = new Sprite ();
sprite.graphics.drawRect (0, 0, 100, 100);
They also use Webpack, which together with NPM makes integrating external code easier, uses local installs of Haxe, and supports hot-reloading, so
npm start -s launches a browser window, recompiles your code automatically when changed, and refreshes the browser window, too. This alone is nice for development.