What you need to know about CAPTCHAs is that they're image files with a value associated with each one. When you enter the characters you see, a value gets calculated and compared to the value of the image object. They aren't randomly generated. You could do a CAPTCHA object filter of just images that contain Qs and import those into the rotation, thus increasing the rate t which Qs show up…law of averages, my friends. Nothing special about it other that the site operator artificially increasing the number of CAPTCHA images that contain Qs.
…case in point. The attached is the CAPTCHA displayed when I was posting this post.