Perhaps that has to do with how careful they are with giving out their email addresses. Some people don't think twice before putting down their email addresses, even when the website looks dodgy. Also, mail-forwarders (jokes, health info, blah blah) are more prone to have their addresses collected by spammers (along with their full name). If an email has your email address and name right, then it's going to pass through any spam filter unless it contains words that will trigger the content filter (p o r n, free r o l e x es, etc).
On average, I get almost one hundred spam a day because I forward two ISP email accounts to my Gmail. At most one or two of them make it into my inbox. Besides the Mac OS X Mail I haven't seen an email client more efficient at sorting out spams.