There’s “DNS Settings” window and it contains button “Cache”, which opens “DNS Cache” window, where you can see existing entries. And this second window has “Flush Cache” button. So you can flush it right there, from the same window.
But then there’s “Web Proxy Settings” window and it contains button “Cache Contents”, which opens “Web Proxy Cache Contents” window, where you can see existing entries. But unlike with DNS, there is no flushing button there. It does exist, but it’s in different window.
=> It’s same thing done differently for no good reason. It’s no biggie, but it’s true that it would be more consistent if proxy had it done the same way as DNS.