Because of a lack of specifics, I can't answer this very general question in a meaningful way, nick_irvine.
Honestly, the best way to get an answer to your question would be to contact AdWords support - where you can be specific and provide all the needed details as to which ads appear to be 'sneaking through the filters', etc.
In the absence of specifics, I'd guess that:
* You are seeing unreviewed ads, which will stop running once they've reviewed (and disapproved).
* You are seeing ads which are within the guidelines and/or policies, based on information unique to the particular account(s) involved.
One common example of this latter point would be an advertiser who seems to be "unfairly" allowed to advertise with a protected trademark in their ad copy (which others are not able to do) - but who may do so because they have been specifically allowed to by the trademark holder - for a reason that may be clear only to the trademark holders themselves.