Why is part of the order displayed under Pre-Order items in the cart?

It means that the product is oversold by it's available quantity. Please refer to the following on how this can occur.

It means that the product is oversold by it's available quantity. Please refer to the following on how this can occur: 

If you had enable Pre-Order for the items and there are still available stock in your store:

In the Products page, the "Continue selling when out of stock" option is checked and the available quantity is more than zero:

If you purchased more than the available quantities, part of the order will be reflected under "Pre-ordered items" in your cart:

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Learn more about different pre-order statuses here.