Could it be that the pre-selling is partly because Compassion has to be paid before a certain date each month and the superblock has some variation in terms of it's date?
Yes, I believe this is true. Being that months in the year is anywhere between 28 to 31 days, I can see the Compassion sponsorship and monthly distribution will have flux.
You can still sell BBP after you receive it without ever involving your personal wallet addresses. You can also build a buffer like BiblePay has already done with Compassion. The buffer is there so you have some wiggle room for BBP volatility, and other external factors such as monthly superblocks and # of days in a month change.
I want to be clear that even though Rob makes it personal, I've never insulted in personally. I only criticize the process and wish to bring more people to BiblePay by having a process newcomers can understand and trust. Rob's a brilliant guy. I wish he made an effort to receive criticism with an open heart and play nicely with others.