Difference between npm run watch and npm run watch-poll
watch will listen for file changes, however, on certain systems this won't always work.
watch-poll periodically checks (polls) for changes e.g. every 1000ms it will manually check to see if any files have changed.
watch-poll is an alternative to watch in certain enviroments watch might not track changes properly, therefore watch-poll was implemented.
Poll will check the files every x seconds rather than automatically picking up on changes through watching.
You can read up on the docs for a more information about mix.