It was the behavior you describe which I found funny, not your comments. Nothing to apologize for. It’s been a while since I worked with response codes but I think somethng in the 300s is the response from the server if a cached file has not been changed on the server. The file is not sent down if this is the response.
Browser caching is a way to limit unnecessary bandwidth and reduce latency for a site that has hundreds or thousands of requests per minute. The more resources such as script libraries, style sheets, images, audio and video files, etc. that can be cached, the faster each page load or repeated visit will occur.
When we F5, all changed files should download. If in your case it only happens when you flush the cache first, then I have no explanation for that. One could speculate or surmize it has something to do with the server, or variant browser behavior, but that is just conjecture. If you find out more about this topic, do please bring us up to speed with you discoveries.