Steam bomb, Gravel worm and Mech spider from khazdhuli collection only have 445 total stats.
Most minions have 450 total stats, and dragonettes have 525.
Dark hound have normal stats.
This make them a little bit inferior compared to other minions. I’m not sure its intentional or just a overlook, so I choose to post it as a bug report first.
Hey @Hagane
I’ve raised this with the Development Team to see if this was intended or not and They have confirmed that this was the design for the Steam Bomb, Gravel Worm and Mech Spider.
OminousGMan - Support Human 
Seems to hold true for items as well. The seasonal boots have the same effect as Elven boots but just for different mana colors…yet at the same rarities they produce fewer bonus gems for no discernable reason.