This is because the inventory numbers associated with the Bundle product are generated on the fly from customer interactions with the bundles. They don’t accurately reflect your actual inventory, just the potential available number of bundles at any moment in time.
The app will track your inventory at a product level if you have the inventory policy for products set to Shopify tracks this product's inventory. An example below for reference.
In this instance, the app will deduct inventory following a purchase, and if the order is returned or cancelled, the inventory will be added back on to the product.