Steam is allowing developers to create dynamic bundles, which get cheaper if you already own some of the games contained within.
Steam bundles are a great way for developers to sell a collection of games at a discount, and a great way for gamers to purchase a whole bunch of games on the cheap. The only problem is when you see a cool bundle, but realize you already own a few of the games in there. Valve is aiming to change that with dynamic bundles - bundles that will get cheaper if customers already own some of the games contained within.
"With Steam Bundles, if a customer already [owns] some items in the Bundle, they will pay for and receive only the items not already in their account. This allows the best fans of your series or franchise to 'complete the set' and get a deal on the remaining items in the Bundle.
"Past Complete Packs were sometimes a bad deal for customers that already [owned] one or more of the products in the pack. Either it made bad economic sense for those customers to purchase the pack, or they just felt bad about doing so since it [looked] like they were paying for products they already [had]. The new Steam Bundles system addresses this."
Keep in mind that this is a totally optional feature, and is decided on a publisher by publisher basis. That said, the Headup Games Complete Bundle is already using the new system, and we can probably expect quite a few more to follow.
Either way this is a move that is pretty great for both publishers and consumers alike.
Source: PC Games N