Captcha and Usability

You can argue about the need for a captcha, about its advantages and disadvantages. But there is one question that does not bother me:

Why, when I enter the password incorrectly, but correctly enter the captcha, they force me to re-enter both of them? After all, if I just correctly entered the captcha, I have already proved that I am not a robot. Why should I prove it again?

The only exception I have encountered so far is the mininova.org torrent tracker.