User blog comment:TZ'sAki/AW SR Drop Probability/@comment-25067771-20140814075624

I assume that event AWs and FAWs have some increased rate at which you recieve them in comparison to non-event SRs. Probably something like 2-3x more often than non-event SRs.

As for the rate at which SRs drop in general... well, I dunno, it seems completely random to me, though I recently have had an excess of Tensen NyanNyans. A friend who's just as active can't seem to find a 2nd copy to HSR her for his life though.