Cats leaving small bits of poop near litterbox

Hi everyone, my cat uses the litterbox consistently however once in a while he leaves small piece of his poop around the litterbox. I'm not sure if it's accidental or there's a medical condition but he seems healthy to me: eating with no appetite problem, grooming himself, very energetic when playing.. any idea?

Hmm, is the poop really hard? I'm wondering if he's constipated, and a little piece gets stuck and falls out a little later after leaving the box?

Sounds like a left over. If you suspect constipation then additional fiber would be helpful.
 

I'm not really sure if he's constipated because he usually poop when we are sleeping and again this only happens once in a blue moon