An order number is the number of the order that the user placed. When an order is confirmed, the system is able to generate an invoice for the order and the invoice has its own number, different from the order number (it's a requirement in a lot of countries).
If you want to have the same numbers, then simply configure the order number format and order invoice format options of the configuration to display the same thing.
For example, you could have the order id as order/invoice number. In that case, simply use the tag below on both options:
{order_id}