It probably has alot to do with availabilty of students being out of school. Maybe some teams are planning on back to back regionals and cannot get thier students out for 3 weeks in a row(not a good idea). Some also might be planning on week 1 then 3 then 5 or something like that, but it also could have alot to do with teams budgets and how far they plan to travel and there is nothing in week 4 within thier range of travel selection. I think that there are a ton of factors on why there are more in all of the weeks but 4, just my opinion.
