I summoned a Common pet from a High Grade Summon, how is that possible?

The Common pet is actually the second Grade for pets. This is because the grading starts from Basic > Common > Uncommon > Rare > Epic > Immortal. As a result, in a 2-4 High Grade Pet Summon Scroll, you should expect to receive pets from the grades Common, Uncommon, Rare and Epic.

Note: While the basic grade exists, there are no basic grade pets in the game.