OEIS sequence A272573 describes a sequence generated in the following way:
Start a spiral of numbers on a hexagonal tiling, with the initial hexagon as a(1) = 1. a(n) is the smallest positive integer not equal to or previously adjacent to its neighbors.
The sequence begins 1,2,3,4,5,6,7,4,6,8,....
If you look at the scatterplot, there appears to be a very sparse region. 
Question
I'm curious if there is some heuristic which explains why this gap appears. (In particular, the heuristic should also account for the fact that such a gap does not appear in A260643, the analogous sequence on a square grid.)