If there was a statistic, I would imagine too that the number applicants who have meaningful authorship are much lower: in undergrad its often a result of a professor's generosity rather than meaningful intellectual contributions... much like there are many applicants who do research for only one semester: they're still considered as having done research, but its a far cry from someone whose been with a lab for 2 years.
...Granted this is just based on my own observations, and from a general consensus I've seen on SDN.