>Why are there gaps?
because those are the cells where the midpoint of a and b would not be a whole number. Or, worded another way, the difference between a and b would be an odd number.
>How does that relate to what we are doing?
Fuck if I know…
There definitely seems to be a pattern to them. [e,2] seems to only have gaps where a and b in the cell above it are both odd. So there seems to be a relation to higher cells. If this thing does have a tree structure, which I am not seeing atm, probably means the cells they relate to could be the end leaves. The gaps also have reoccurring patterns both vertically and horizontally.