Cached pages are categorized as Anonymizing Utilities since this cached datas allowes to access data which might be blocked normally by other categories.
Example. http:/www.sex.com is blocked by your policy due to category sex.
If someone fetches the cache in google he is able to do this since google is categorized as Search Engine....
So accessing this cache will bypass your policy. To prevent this google cache is categorized also in Anonymizing utilities.
If you want to allow this you should a new rule before your categorization ruleset take place.
The easiest way might be to allow google generally.