Why Scatter signals reach the receiver by many small ionospheric or tropospheric scattering paths at once, and those paths vary in length and change moment to moment. The many components add and cancel with slightly different delays and Doppler shifts, so the received signal has a rapid flutter or watery, distorted quality. Scatter signals are also typically weak and the audio is hard to copy.
Watch out The choice about high intelligibility is the opposite of reality, since multipath distortion makes scatter phone signals sound rough and hard to understand, and scatter is not a nighttime-only mode. Very large sudden swings in strength describe other fading mechanisms, not the fast flutter of scatter.
Scatter = many scattered paths = flutter. Weak, watery, hard to copy, not crisp.