This is just a guess, but maybe the reason LSN seems to indicate that most acceptances had come out by March last year is that many of the LSN participants sent in early apps (which is not true of the entire applicant pool) and, as a result, received decisions accordingly. Also, keep in mind that LSN is only giving you a glimpse of the overall applicant pool and, in my opinion, one that is skewed toward the higher (better numbers) applicants who would tend to have decisions sooner rather than later.
Also, if you check out many law school's websites, you will find admitted student's days in both March and April. This implies to me that some students will not find out in time for the March date, yet are accepted in March in time for the April one.
Take what you will from this, though, because I am still waiting on three apps and my optimistic post helps me feel better!