Rebuildable work orders having parent wip entity id as null in wip_discrete_jobs table

Rebuildable/repairable work orders are generated from the maintenance work orders and usually will not have the asset number associated to them. and they will have a associated parent work order.But i can see many rebuildable work orders which do not have asset numbers and also the parent wip entity id being null. 

I want to understand when the parent wip entity id on wip_discrete_jobs table will be null for these rebuildable/repairable work orders.

Note: there is no record existing in eam_wo_relationships table as well for these work orders.

Thanks in advance.