When the item grouping is selected as ‘grouped’, the bundled items are grouped under a parent line item. When this type is enabled, the subtotals of the parent/child products, as well as the total price of the bundle is calculated which also includes the base cost and sale price of the bundle. Whereas, when it is selected as ‘flat’, bundled cart/order items include a reference to the bundle they belong to. If you have added a base price to the parent product, then all the prices of the item will be displayed in a segregated manner. The total bundle cost will be the total base cost and the child products will display their own individual prices.