Lightroom weird cloning issue

Hi

I have a strange problem with cloning in Lightroom. It's 4.3 but it happens a couple of times in 4.2 too. Sometimes it works fine but occasionally this happens. I use the clone, but instead of cloning with 100 opacity, there is some of the original part showing through. It is also not exactly even in how it shows, but fading towards the edge of the circle. If I change to the healing tool the problem goes away, but it's healing, not cloning.

Maybe I'm missing some setting and don't really know how cloning should work? I thought if it's on 100 opacity it should simply clone it i.e. replace the part with the second circle. Maybe not?