I don't have data to prove it, but I will bet that the drop rate is variable. I'm guessing it's probably like the rate of chance time on extra stage. It probably fluctuates according to some set schedule that we are not privy to as opposed to being a flat x% rate all day. This would help explain why she tends to appear in 'clumps.' I've noticed other people say that they got her back to back, too. We can know as much about the base percentages as possible, but I'll bet almost anything that everything is variable according to a master schedule.