TransactionScope

hi experts, I got a wired problem need your help. I have nested transactions. The outer method calls an inner method SaveChangesToSource that opens another transaction scope. Actually I know my underlying sql may have problem. What I cannot understand is why my inner transaction complete successfully and it looks like the exception is been postponed until I call outer cope.Complete??it's like the inner transaction does not report error until you call outer scope.complete.. then the the outer scope rollback the whole thing...??

using (TransactionScope scope =newTransactionScope()){SaveChangesToSource(changes);
      scope.Complete();}protectedoverridevoidSaveChangesToSource(IEnumerable<IChange> changes){
    using (TransactionScope scope =newTransactionScope()){
         _bl.SaveChanges(changes);
         scope.Complete();}}