It's due to how technology is improving and how your hardware compares to the latest. See the quote below which is what you'll see when you click "What do these numbers mean?"
|What is the Windows Experience Index?
The Windows Experience Index measures the capability of your computer's hardware and software configuration and expresses this measurement as a number called a base score. A higher base score generally means that your computer will perform better and faster than a computer with a lower base score, especially when performing more advanced and resource-intensive tasks.
Each hardware component receives an individual subscore. Your computer's base score is determined by the lowest subscore. For example, if the lowest subscore of an individual hardware component is 2.6, then the base score is 2.6. The base score is not an average of the combined subscores. However, the subscores can give you a view of how the components that are most important to you will perform, and can help you decide which components to upgrade.
You can use the base score to buy programs and other software that are matched to your computer's base score. For example, if your computer has a base score of 3.3, then you can buy any software designed for this version of Windows that requires a computer with a base score of 3 or lower.
The scores currently range from 1.0 to 7.9. The Windows Experience Index is designed to accommodate advances in computer technology. As hardware speed and performance improve, higher score ranges will be enabled. The standards for each level of the index generally stay the same. However, in some cases, new tests might be developed that can result in lower scores.
I am particularly interested in the last paragraph.
So when you get the latest driver, the score can get updated to reflect how your hardware compares to the most current hardware supported by that driver (at least, this is the way it seems to be working). So I mean I think that if you were to use an older driver (particularly one that doesn't support the 300 or 400 series), then the score might go back up to where it was.