ChatGPT-4 message limit: how it's calculated and how to bypass it

I have a strange situation with ChatGPT-4 message limit. On my main computer, I can only send 40 messages in three hours before I get the error message “Too many requests. Please try again later." However, when I use ChatGPT-4 on another computer with a VPN, I can send messages again, even though I’m not waiting.

Why is this happening? How is the ChatGPT-4 message limit calculated?