Service worker example triggers "offline mode" when actually it's a "confirm form resubmission"

This example code:
https://github.com/GoogleChrome/samples/blob/gh-pages/service-worker/custom-offline-page/service-worker.js
works very well normally, to give a custom error page if (and only if) the application is offline.

However, in the case where I post a form submission, then post another one, then back (i.e. the back buttons hould takes me to the result of a previous post), then the SW tells me I've lost connection to the server. In other words, it's mistaking the "ERR_CACHE_MISS" / "Confirm Form Resubmission" for "ERR_INTERNET_DISCONNECTED."