All our cats have done and, those who are alive and living with us now, still do this. I researched this behavior a while back. If I remember correctly it was explained that cats (the big ones our domestics are descended from) always "clear" the water, checking that it is suitable for drinking. Remember, in the wild a water source may be covered with algae or other natural debris. If the cat can "clear" the debris away and finds fresh water, it will drink. It may not be needed now that they always have access to fresh water, but they still do ii. Aren't our brains wonderful? Theirs, and ours, are primitive and sophisticated at the same time. I love my cats!