Older siblings are shaped by parents into perfectionists to set an example for the younger siblings. They are less likely to do bad things and more likely to turn out better than the younger siblings.
What it also says to me is it is effective on the older siblings. If the parents maintained the same level of strictness on the younger siblings, then the younger kids would turn out better as well. Because the parents are only putting on act, the younger siblings end up more likely to become screw ups.