The best thing is to ignore the 1/6, 2/6, etc as it just confuses everyone, basically if NRC knows they have 60 of a designator to fill each year and will have 6 boards they will divide that to pick 10 each board. The number of officers they need for each designator generally doesn't change much each year, so that number might change slightly, but they remain fairly similar year to year to year. What can change things is how many are picked for ISEL or URL ISPP, the extent of that effect is known only to NRC, and that could be a reason why they do cancel some boards or it could be a combination of other factors.
April is about when they start filling the next FY, but it has started to be filled as early as January on occasion.
Don't think of it as a person picked in one FY or another since that doesn't matter at all for OCS programs, it is not when a person is picked it is when they commission. I had a candidate selected in a Sept board, went to OCS in late July and commissioned in Oct which means he covered 3 FY's but the only one that mattered was the one he commissioned in.