So, Wirecard claimed to make huge profits. Now, the auditors would expect to see a pile of cash in the accounts. However, Wirecard claimed to expand rapidly by purchasing other companies in Asia. Those, then, booked most of the "profits" and were the assets on the book. Wirecard produced bank statements from the Philippines claiming that they had $2bn cash sitting there. So, to the auditor, the numbers added up, and the whole story was somewhat coherent. It's just that the foreign businesses and that cash didn't actually exist.
Which, to be clear, is a failure of the auditor. We don't need auditors to make sure the numbers add up; the whole point of double-entry bookkeeping is "the numbers always add up".
