Forwarded it. BTW, here is my explanation (just my thought on thunder)
There are three divisions of distance for thunder irrespective of modifications.
Max splash damage radius = 1m (maximum splash damage irrespective of no.of.tanks within the area)
Average splash damage radius = 5m
Min splash damage radius = 12m (after it no splash will affect a tank)
Average percentage of dealing average splash damage = 90%
So, in your query, it indicates the average splash damage dealt when the tank is only 1m away (or even nearer) from projectile
It means thunder has 90% chance of dealing average splash damage within 1m mark, rest of the time, it deals full damage (10%)
This is me so far using thunder all day. Hope you understood ;)