The ProxySG or Advanced Secure Gateway (ASG) may cache a URL category for a period of time before it gets removed or updated.
Most Webpulse responses are cached for 10 hours. If a site that has been categorized as "None" and previously cached then receives a Security category (i.e. phishing, malware, etc...), or if it is categorized through a paid priority rating contract, then the new rating will go out in the security update of the DB (~5 min) which is checked prior to the Webpulse cache and should get updated quickly. If a site is not a security category then the updated category will in the next differential update of the DB (~4-5 hours).