What I also observed was that three of the surges were particularly common, while three were somewhat rarer, and three of them were substantially rarer, in both tests. The last surge, mephits, was rare in the first twenty or so surges, and then increased in frequency alongside the decrease in the success rate of Tides to produce a surge at all. This pattern, too, occurred in both 100 surge tests.
I haven't finished reading yet, but I wanted to get this down before I forget.
Did you have weighted dice turned on? My first thought when I read this was that it's possible weighted dice apply to the roles for surge effect, which would explain why some effects would appear more common than others.
