Considering that Demon King Piccolo was essentially a being born out of emotional essence, I like to think a bit of magic was involved as well. Perhaps the other demons that were born had a different style of birth, and him giving birth to Piccolo Jr. was more survival instinct kicking at the last minute.

That is a nice explanation

Piccolo considered himself a demon, so his offspring would be more demonic looking then himself. Piccolo jr was meant to be his alter-ego, so him looking exactly like him was intentional.