Audits provide reasonable assurance, not total. When auditors test access controls for a homegrown application for example, it is unreasonable to ask that a full code review is done to check 100% that checking the box next to Admin confers that, and that checking Read Only restricts it always. In my experiences performing these tests (as a young grad who had never worked on a software dev team), we would ask what the permissions were designed to provide and limit, and observe in the system that they did that. If a developer had programmed a backdoor that when you press A+B+3 and whisper into a microphone grants unlogged admin access, our test would miss that. But that's why we also test change controls and who has access to push to live, etc.

Edit - and to speak more to the topic at hand, there were plenty of people at the firm I worked with who absolutely had the technical expertise to perform such an in depth audit. They are simply engaged when higher levels of assurance are required. What level of scrutiny should your auditors provide your bathroom time monitoring system?