Trunks and Goten where born after their fathers were able to go super saiyan, which if I remember correctly initially was described (from a lore perspective, but I could be misremembering) as being somehow a 'condensed' form of going great ape, that is to say that super saiyan was the great ape power-up, without the great ape.

We can rationalize it as: If parent has ever gone super saiyan before birth of child, no tail, otherwise tail.