I imagine the data is true overall. However, I'd guess that earlier applicants are, on average, applying to more competitive schools. Many of the top 50 don't even accept applications after early to mid-March, so those 20% are likely applying to a greater proportion of lower ranked schools than the 40% who apply by early January.
I'd be interested in seeing the data broken out that way.
Regardless, it's most beneficial to apply as early as you can compile a strong application.