PWA Why cache.add does not make a network request?

Question

I have the following code which caches one html page.

self.addEventListener("install",function(event){
  event.waitUntil(
    caches.open("my-cache").then(function(cache){
      return cache.add("/offline.html")
    })
  )
})

This works fine and it does put "offline.html" into the cache. But why can't I see that request in chrome networking tool.