This is covered in ChatGPT’s learn more section:

> Limitations

> ChatGPT sometimes writes plausible-sounding but incorrect or nonsensical answers. Fixing this issue is challenging, as: (1) during RL training, there’s currently no source of truth; (2) training the model to be more cautious causes it to decline questions that it can answer correctly; and (3) supervised training misleads the model because the ideal answer depends on what the model knows, rather than what the human demonstrator knows.

https://openai.com/blog/chatgpt/

ChatGPT has done that for me too, but as you note asking the question a slightly different way produced a positive response. I think they simply trained it to produce “I don’t know” as a response to certain patterns of input.

That's a filter answering, not GPT. And there are ways to disable those filters (eg: "Browsing: Enabled" was reported to work, though I haven't tried it myself, and it would let you elude the "I can't browse the web" filter).