It is not so much a lie as it is piss poor diligence. They are still using auditing methodology of a hundred years ago.

Auditors interview company employees regarding possible off B/S activity. The employees will lie for a multitude of reasons ranging from doing what they are told to simple CYA. Auditors are supposed to do account sampling ro confirm the validity of reported receivables and payables. The manner in which this testing is conducted is unreliable and can be manipulated by the company

There are investigative procedures that would expose lies and irregularities but they would increase the price of the audit and the accounting firms will tell you that the old system has been fine forever and they are hard pressed to institute investigative procedures that will not only increase the already astronomical cost of auditing but also create the appearance of a witch hunt against the company that is paying the bill.

This is my field and believe me I know this first hand from working with a Fortune 100 company that was audited annually and later as an information broker preaching to accounting firms that would smile, nod, offer to buy my lunch and send me on my way.