Cleave on the mage won't affect the pet, sadly.
Since you have most of the relevant crit buffs (Sune) available (and assuming your eyeballing of crit rates is extremely accurate

) I have only one more idea to put forward:
The other pets don't have a better crit rate, but are critting more because they are swinging way faster than your pet.
How? Bard overhaste. It's just you and a warrior, so your pet is probably chugging along at 85% haste. With a bard your pet would be north of 120% haste.