So the only difference being that you don't receive full credit for the price on steam?
You still receive full credit, as long as they're 20% of your total contributions or less. For example, let's say a user has contributed $200 in games not from a bundle or before the bundle took place. 20% of $200 would be $40, meaning they'll receive full value for $40 of possible bundle games they've previously posted, or decide to post.
The order doesn't affect the result, so even if they submitted $40 in bundle games to start and didn't receive full value, but then decided to post those $200 in non-bundle games, they still end up with the full value of $240.
In short, the vast majority of users will never see a reduced value whether they're posting bundle games or non-bundle games. It just acts as a fail-safe to keep users from reaching high contributor values by only gifting bundle games.