I've chased only one mega. It was at 195 and came off quick at 199. That was early on and before I had figured the mega frequency at about 1200 because of how often the mega symbol was landing.

I had put the mini frequency at 221, maxi 663, mega 1157. When the par sheet numbers came out showing 220, 675, 2553 I had been averaging mini 214, maxi 641, mega 2440. So my calculations were right on top of mini and maxi but I knew something was up with the mega before the par sheet numbers came out.

I think Mission has the best explanation. If a bonus symbol doesn't come out on the first reel then the 5th reel is different. Or if a bonus symbol lands on the 1st reel but doesnt land on the 3rd reel the 5th reel is different.