Those are typically caused from "speckles" in the original bitmap. Spikes are easily sculpted out using the Smooth (Lower setting) and/or the Remove tool. Further blending sculpting may be required. If the spikes are actually holes in the model, then turn off the preserve transparency and use Smooth Raise or Deposit or Smudge Raise to fill them. (There are other techniques to fix things like this such as merged elements etc, but sculpting is usually simpler.)
You could also consider editing the speckles out of the bitmap before importing, but you'll be working at a zoomed in pixel level and could take some time. Global speckle removal tools are not recommended because it will often yield a "melted chocolate" appearance (less defined detail).
Was this a greyscale you found or something you modeled yourself from a 2D photo/bitmap?
Not sure why the difference between the carve strategies, but I think the model itself is causing this. You can save it out as an STL and import it into the free MeshLab software to see if it shows defects there also (and make repairs if necessary). If no defects show up there, then it could be a display issue in the toolpath preview (unlikely, but possible).
-ShopBot Buddy PRSAlpha CNC