Pobox uses a series of conditions to identify messages as spam. Those conditions are noted in the Spam section for bounced messages, and in the message headers as "X-Pobox-Antispam:".
When you release a message to yourself, we make note of which condition it was caught by, and use that to determine how accurate it is. For details on how accuracy is used to segment the conditions into Weak, Standard, Aggressive and Very Aggressive groups, please see the spam slider help page.
We also use by-country blacklists, to help people reject all mail from areas of the world that they know they do not have legitimate correspondents in. Because each person has different areas where they don't receive mail, we cannot create a system-wide "accuracy" rating for those blacklists, and you must turn them on yourself.