What happens when you overpower the grid?

When you have a full grid and you buy some "+1" power with a star, it's called "Overpower grid" and it says :

"Your Power Grid is full. Additional Power will permanently upgrade your Grid Defense" which I understand.

But it also says: "The bonus will decrease the higher your Grid Defense gets, to a maximum of +25", what does this mean? What will decrease exactly? How can it reach a "maximum" when it's "decreasing"? Is it a typo?

Solution 1:

The Grid Defense starts at 15%, and can be increased by gaining additional power after the Power Grid is full. This is called Overpower Bonus, and can get as high as an additional 25%.

Up to a 10% Overpower Bonus, each point of overpower will increase the Grid Defense by 2%. After that, each point of overpower will increase the Grid Defense by 1% instead. This is how the bonus decreases the higher your Grid Defense gets.