My powerful cards are bottom of my deck

I play a Mage and every game now, and Im going over 20 or so, I wait for 2 certain cards Mecha-Shark, and low and behold I never see them. As a Priest stormed through my deck today, they were both 29 and 30.

Is this a bug or are certain powerful cards assigned a lower % ‘spawn’ rate that I don’t know about? Just seems too coincidental to me.