Why is there a difference between SEC filing date and a Date of Event for an SC-13D form?

Companies are required to file a Schedule 13D within 10 days after the purchase.
SEC.gov: Schedule 13D

It is not clear that that is the same as the "Date of Event which Requires Filing" is the same as the date of purchase, nor if there were other reasons why it did not need to be published within 10 days of July 23, 2019.