SpamAssassin Stops 35% of Spam

I did a test over the past 36 hours to test how well SpamAssassin is catching spam on my email addresses. I turned off the "Auto Delete" and configured my "required_score" to 3.0. I then set a rule in Outlook to toss any email with [SPAM] in the subject into my Junk E-mail folder. Any junk email missed by Outlook I tossed manually.

After 36 hours, the email marked by SpamAssassin accounted for only 35% of the total volume; it allowed 65% to pass undetected. This is from a pool of 562 spam emails.

Shouldn't SpamAssassin be much higher in its detection success?

Thanks,
Dave