https://www.youtube.com/watch?v=NbInZ5oJ0bc
Even after sharing that, I must admit, it sometimes feel there is something wrong with the RNG in SW-GoH.
And away from real world the RNG calculations in games (based on seed and the formula to calculate the results) can be really poorly done, and will get the same results more often than not.
Not to mention we don't even know what the chances are to drop certain gear/mods and in Arena for "us" to hit certain effects and "AI" to hit certain effects are.
All I know is it is much rarer to get Arrow and Triangle Mods than others.