I cast black manacles 14 times this week in GW, 0 devours. Ai cast kraken 7 times and got 5 devour triggers in one battle. Percentages are just stupid.
Sometimes you’re lucky, then there’s other times 
Kraken however does have twice the chance to devour.
Take four white marbles and one black, place them in your pocket, how many times will you pull out only the black one? That’s Black Manacles chance.
With Kraken, there are two black marbles and only three white. Bit of a difference between the two.
Ok on purple day I cast kraken 9 times and got nothing as well.
LIke I said percentages are stupid.