Each subresource used on your page (images, JavaScript, CSS, etc.) results in a new HTTP request. The routes that you register will be matched against the URL for each HTTP request. Right now the route you've registered will match the specific URL pathname '/about.html'.
If you'd like to cache absolutely every same-origin request that's made by your webapp, you can adjust your routing logic to use a RegExp wildcard, like:
workbox.routing.registerRoute( new RegExp('/.*'), new workbox.strategies.NetworkFirst() );
You can further modify that RegExp if you'd like to narrow down what gets cached, or use a different strategy for different types of URLs.