Prior to September 18th, 2017, requests to GET /admin/orders.json including the since_id parameter would return a list of orders sorted by created_at. This would present problems when multiple orders were created within the same second, as results wouldn't dependably return in the same order on subsequent requests. Going forward, orders with the since_id parameter will correctly be sorted by order ID, even if the orders were processed at identical created_at times.
If you have any questions or concerns, don't hesitate to comment in the thread below!
All the best,