Blending modes like Current & Below just work in a different way than Current Layer. We cannot make it the same because of some math limitations, sorry. In Current mode we both sample colors/alpha from current layer, and paint also on current layer. But in other sample modes (All Layers, Current & Below etc) we sample from other layers, but paint on current layer. That doesn't allow to get as strong blending effect as in Current Layer. It is a bit hard to explain it clearly, sorry. But it really works, just the smudging effect is not strong.
