Why is a Fortnite Bundle in the Item Shop discounted from its initial amount?

This happens when you already have or have bought a part of the Bundle. If you want to purchase the remaining cosmetics in the Bundle, you will see the amount of V-Bucks minus the cost of the cosmetics that you already have in your locker. 

Notes: 

• Bundles are offered to you at a discounted price when you purchase all the items at once. If you already own a cosmetic that is offered in the bundle, you may see a further discounted price for the bundle in the shop. However, as individual items are not offered at a discount, purchasing all items individually will not necessarily provide further discounts on the bundle. 
• The discounted bundle prices are only offered to the Epic Games account that already owns the item(s) and will not affect the price for sending gifts. The cost of the gift bundle is always shown in the gifting window.

Example: Before purchasing the Pickaxe from the Bundle:

After purchasing the Pickaxe from the Bundle: