Web Inspector is typically presenting network requests and responses as exposed by XHR/fetch.
It would be more appropriate to expose the requests and responses as done at the network level.
I just tried running the following test: https://github.com/w3c/web-platform-tests/pull/3592/files
Looking at firefox/chrome inspectors, we see that the second response is a 304 response at the network, that translates to a 200 response at JS level.
The corresponding request also has specific cache headers, since it is a conditional request.
Doing the same test on WebKit shows that the two responses are 200 and the second request is the same as the first one.
Doing a quick wireshark test, the second request sent on the wire is a conditional request and the response is a 304 response.
The low-level network information is much more interesting for debugging than the current exposed one.
IIANM, the same principle applies for redirections: they are not exposed by web inspector while it is very useful debugging information.
See also related bug 160286: headers filtered out in a response do not get exposed by web inspector. This makes it harder to debug.