The usual sources are consistent: subcontractor payments that were never treated as payroll, employees carried in a lower-rated classification than their duties support, payroll reported net of includable items, and operations added mid-term that nobody disclosed.
None of these are exotic. They are the ordinary consequence of pricing a policy on estimates made twelve months before anyone knows what the year looked like, which is what the audit exists to correct.
Why it matters at audit
As rate levels fall, leakage takes a larger share of the margin. The same error is more consequential in a soft market than a hard one.