That's true - I looked into why even though there's no cache-control header: when there's no explicit cache header, some browsers will cache anything with a Last Modified header for some small fraction of the distance between the current date and the header.

Here's the chrome code for that:

    https://chromium.googlesource.com/chromium/src/+/49.0.2606.2/net/http/http_response_headers.cc#949

As this is a static site, those Last Modified dates are probably coming from the fs under the webserver. Obviously for dynamic sites the Last Modified header won't be there unless you put it there.