Spam might show up in my inbox a few times each week, while I need to remember to check several layers of junk mail filters for false positives legitimate email marked as spam by an aggressive filter.
Try counting sometime the number of commercial messages you encounter in just a few minutes of internet surfing or sifting through your email everything from Google ads to banners, flash videos, email blasts, spam that slips through your filter, and e-newsletters you now regret signing up for.