Madcow, you seem to have several things wrong there.
First of all, are Wyches only WS4??
The Wyches are assaulting the Orks, not the other way around.
The Wyches have 1 attack base, +1 for two weapons, and +1 for charging, so 30 attacks total, not 20.
You didn't factor in their pistols, which, well, I suppose they could have Fleeted.
And don't the Wyches have some crazy rules in CC?
The Orks, attacking back, only have Strength 3 when they don't charge, so they're wounding 50%, rather than 66%.
So basically, there should be 6.something dead Orks, and SIGNIFICANTLY fewer Wych casualties.
the Wyches get the charge.
And plus, working out mathhammer in a vacuum is always a bad idea.
There are so many other variables in the game that you have to take into account.
What if there is a bigger threat on the board?
What if their transport gets shot?
What if the Orks have a transport?
What if the Orks have FNP?
Take into account the Power Klaw too?