Off-topic, I am an accountant at one of the big4 firms:
In accounting, a term called materiality is employed. Materiality basically means: the accountant will perform statistic sampling methods to gain 95% assurance that the financial statements do not contain a material misstatement.
This is done because auditing all transactions at a large company during a year is simply impossible, or at least impossible in time for the financials to be still relevant.
So 1 auditor might perform procedures and sampling and come to the conclusion that the financial statements represent the factual situation, while a 2nd might have slightly different findings.
All financial audits use this principle, therefore I would imagine that the FIA has used the same 5% rule.
Costs and revenue are more subjected to estimation or subjectivity than you might think