I don't think it was as much as two and a half years ago, but yeah, the ratio of explode mana generation was lowered to 70% per gem a LONG while ago on PC / Mobile
Calculation details :
Damage generation is given by 1*skull exploded
Mana generation for X mana gem exploded of color C is given by round(X*70/100) C mana
Each mana color has it's own mana generated counter
In practice, this means that exploding 3 gems of color C will give round(3*70/100)C = 2C
While exploding 2 gems of color C and 1 of K will give round(2*70/100)C + round(1*70/100)K = 2C + K
So, if your unit is using both mana C and K, it's better to aim for 2C and 1K than 3K.
I hope this helps regarding
This problem of yours.