I really think it's just pure chance. Lady Luck works in mysterious ways. And I have a lot of evidence to back this up.
My brother started playing about a month after I did. We each only play our 1

per day, every day. His best level and best score have consistently been barely lower than my own (only this week he managed to one-up me at last, with Tuber's Bag and a shocking amount of determination). But listen to this. He has BoS, I don't. He has 2 BoM, I have 1. He has 2 Tuber's Bags, 3 or 4 Candy Canes, and an Earthworm ... And I think the only item I have over him is a Pear Egg.
One of my friends got the BoS on his very first day after signing up. I have yet to see it. I'm not complaining, of course; just pointing out that it all depends plainly on how lucky you get.
(I wouldn't know about droughts giving you rarer items, because I have made a point never to miss my game for the day.)
