WebPageTest shows no static cache but I got 304 response in repeated run

I am testing my webpage with webpagetest.org

On my page, there are a bunch of images. I can see them well cached in the repeated run: (304 response is marked as yellow in WebPageTest waterfall result)

However, in cache static content, there's no check on those resources

I found the difference is that those scripts and styles have cache-control: max-age=2592000, while those media resources have cache-control: max-age=0 in the server response. Does it mean that WebPageTest will neglect these responses with max-age=0 in static cache checking?