Not even sure what explanation you're looking for. Asking me to explain why we've had some bad luck is like asking me when exactly we'll find our next block... neither of which anyone is really capable of answering definitively.
"It's just normal variance" is a perfectly fine explanation, as long as you can show that falls in the normal range. The 40-day/50% explanation is a plausible one.
I admittedly was being a bit of a smart ass with that opening line, no offense intended.

I'm glad the remainder of the post was informative.
-wk