Halley is correct - it's an accounting irregularity.
We finely (extremely finely!) balance the amount of media we're picking up against the amount of media we're dropping on the canvas. At times (because of the way we do the two separate processes of picking up and depositing in different manners) there can be a slight imbalance.
The palette knife is a very special tool because the goal is to let it smoosh around paint on the canvas - it should never really reduce or add to the amount that is there. So even a tiny discrepance in either way causes problems. If we take off a fraction more than we're putting down, the paint seems to vanish too quickly and cant be smeared far enough. If we add even a tiny fraction too much, well you can see the result.
The problem gets compounded many times over if people are doing small circles with the palette-knife, because the stroke is overwriting itself at every step - paint is being sampled and mixed more than once per pixel.
So the short answer is.... Yes, it's a bug - the knife (currently!) shouldn't deposit paint onto the surface. It should just move what's there around.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"