Static versus Dynamic coefficients of friction
When two objects are touching, and there is no movement between them, it takes quite a lot of force to make one of them start to move.
For example, if you put a heavy book on a ramp, you have to tilt the ramp quite steeply before the book will start to slide.
This corresponds to the wheel that is turning normally - if you take a snapshot of the tyre at any point in time, the surface of the tyre is not moving against the concrete. It takes a lot of force to make the wheel begin to skid. The aircraft can be rolling at a high speed and still be able to use the brakes - it is a safer condition, with the wheel less likely to begin skidding.
However, when two objects are already skidding against each other, it does not take as much force to keep them skidding.
For example, when your heavy book in the example above has begun to slide, you can tilt the ramp less steeply and the book will keep sliding.
This corresponds to the wheel that is skidding already - if you take a snapshot of the tyre, the surface of the tyre is skidding along the concrete. It will take very little brake pressure to keep the skid going - much less than it took to begin skidding. The aircraft will tend to keep aquaplaning at a lower speed than it was at when it began to aquaplane.
If you understand this, you'll also understand why automatic brakes try to keep the wheel at the point of almost skidding, but not quite. When the wheel begins to skid, you get much less friction, and braking is impaired.
Hope this helps,
O8