Why CasperJS and browsers show different behaviors with CAPTCHA?

I'm using CasperJS to automate some of my tasks. CasperJS will meet captcha when exploring web pages. However, when I use browsers (chrome and safari) to access the page, I don't need to input the captcha. I try to set the userAgent option with the same value in browsers, but it does not work. I want to know why CasperJS and browsers show different behaviors. Thanks.