This is by design (and implemented as explained in the sibling comments). The reason for the randomness is to avoid returning the same thing all the time. For example if you ask it "write me a poem" you don't want it to return the exact same one each time.
You can also set the "temperature" parameter to 0 to remove the non determinism (though I'm not sure that's exposed in the web interface).
It's not, not in the ChatGPT web interface at least. It's just a chat prompt, that's it. Can't control any sampling parameters at all, not temperature, nor top-P or top-K-based selection, nothing. I guess OpenAI is aiming to keep things simple for now. And to be fair, this choice probably does open the demo up to more users who would otherwise be confused at all the knobs and dials. Although they could just hide it by default or something
You can also set the "temperature" parameter to 0 to remove the non determinism (though I'm not sure that's exposed in the web interface).