Why The skip zone is the ring between the end of ground wave coverage and where the first skywave hop lands, so no direct refracted signal reaches it. What you hear there is scatter: small amounts of energy randomly redirected by irregularities in the ionosphere, by the ground at the far end of the hop, or by the troposphere. Because only a tiny fraction of the transmitted power gets deflected back into that zone, the signal is weak, often watery sounding and fluttery.
Watch out The magnetosphere is far above the ionosphere and plays no role in normal HF scatter, and ground wave dies out well before the skip zone, so neither explains what you hear there.
Scatter = crumbs, not the loaf. Only a sliver of energy lands in the skip zone, so it sounds weak and watery.