I stumbled upon something I found a bit odd. I have 4-child tables of which one has a grandchild table (Years).

All the child tables all refer to the grandchild table for calculations. Initially I made multiple table occurrences of the grandchild table (Years) and linked them to each of the individual child tables. By mistake, I referred to the original grandchild table in one of the other child table's instead of the occurrence, for a calculation. The calculation worked fine. I then deleted all the grandchild table occurrences and all the child tables calculations still worked fine - just relating back to the one grandchild table (Years). I'm surprised this works. Any explanation for this? Is it because they all link back to the parent table? Note that the relationship between the child table and grandchild do not allow the creation of deletion of any records.

I can't tell from the screen shot whether the Child TO's are all based on the same data source table or different data source (base) tables.

If they are all the same, your calculations work because the calculation field probably specifies Income as the "context" for the calculation.

If the are not the same as I suspect is the case, then it works because the relationships, even though they trace through Members to get to Income all correctly match to the right record in Years, this may be a dangerous conclusion to make as it may be that the current sample data works for you but additional values in your tables might lead to different results that are not correct.