They have family.
Probably a few 1000's more escape the work camps in China and Russia and assimilate locally.
This is a situation where all parties have no incentives to make defection public, in fact their incentive is the complete opposite.
South Korea will not publicize defection because of fear retaliation both against any familial ties the defectors might have and against SK proper.
The North won't because it would make them look weak, for the North even unsuccessful defections will be hushed since why would anyone want to defect from Paradise?