Curiosity photographs thousands of rocks every week. It does that for years. You will find rocks with almost any shape you can think of. This one happens to be somewhat round in one direction. From the shadows and the way it stays on what is apparently a slope, I would infer that it is not a sphere, but is actually ragged on the other side.
I'm no geologist (do we have one?), but I don't think this requires further explanation.
If I did have to venture a guess I would say that maybe its porous and thus light. Together with Mars' low gravity it might have tumbled in the wind to dull the edges a little.