Turns out Montessori education runs in 3-year cycles (ages 2-3-4, and 5-6-7, for example). Not all schools make that clear, and, if you put a kid in in the middle of a cycle, there is orientation material they will miss. Without that grounding, Montessori can turn out to be a really ineffective experience. (Source: Was a parent who realized this and had to compensate for it. The 5-6-7 cycle was much more effective.)
I can see how moving 1st grade into the first bucket would help even things out. I think pre-K was often 2 years, though, so the first bucket still covered 3 ages or grade-levels.
