My math is also correct. He is doing a different kind of average. Mine is far more simplistic and easy to understand. I'll show you my math to show what you average each time you shoot.
2 markerlights + buffmander for both units. (BS 5 + reroll everything + ignore cover)
Riptide
12 shots - (1st volley) + (Missed shots)*2nd volley
11.6666666 hits - (12*5/6)+(12*1/6)*5/6
~3.56 wounds - (11.6666*1/6)+(11.6666*5/6)*1/6
~3.56 wounds unsaved - No saves allowed against bull**** of this magnitude!
3 crisis suits
12 shots - (1st volley) + (Missed shots)*2nd volley
11.6666666 hits - (12*5/6)+(12*1/6)*5/6
~6.39 wounds - (11.6666*1/3) + (11.6666*2/3)*1/3
~2.13 wounds unsaved - 6.39*1/3
3.56 vs 2.13 in the riptides favor So basically a wraith knight every 2 rounds vs every 3 rounds.
I'm not really exaggerating though? The riptides almost twice as effective on average. Unless you tell me where my math is wrong, I am going to assume its right.