Logic would mandate that there must be an equal number of points in each UY in order to maintain balance in the system. I suspect it's required by law but that may be a stretch.
I assume the above to be a fairly true accessment of how the system must be set up.
Having said that, I get an e-mail from Jamie at TSS every day, sometimes twice a day. I used to think there were hardly ever any June UYs for sale but I've noticed that they come and go. My gut tells me that it's probably relative to the beginning and endings of UYs and that there's probably a pattern to the madness. For example, more UYs get sold right after they renew each year than are made available near the end of a UY. I don't know that particular one to be true, I'm just using it as an example.
It would be an interesting analysis project for a senior thesis in applied mathematics but I just haven't had enough inclination to want to dig to find out myself. My mathematical training tells me there has to be a system inside the chaos but it's just not interesting enough to make analysis worthwhile as I doubt many people would really care at the end of the day.