Status code ranges are less about fault than they are about determining whether the client can retry the request without changing it. 5xx means that the client

Status code ranges are less about fault than they are about determining whether the client can retry the request without changing it. 5xx means that the client can, because there's a chance repeating the request will succeed. 4xx means the client shouldn't, because it needs to change in some fashion before it will work. This is so that clients that don't understand a specific status code can fall back to the more generic x00 code and still behave sensibly.