It's exactly what the title says. The skulk sensors introduced in the latest snapshot can produce different levels of signal strength depending on the source of the sound they detect if they are connected to a comparator. Falling and walking have different signal strengths (5 and 1 respectively), but if an entity is moving when it hits the ground, the walking sound is made first and that is what the skulk sensor detects. This could be the intended behavior, but it doesn't make a whole lot of sense.