The mechanics of flight.
I believe all flying creature are simply elevated, you can see it from ranger’s pet when they use birds or even when you transform into a flying creature, they actually follow the ground’s pattern (lower when going down a hill, getting stuck from rocks and things like that.
So they’re not flying per se.
For what it’s worth, ground targeting skills seem to hit flying enemies when they’re on top of the targeting zone. Though I’m not sure if that’s also because the range of those attacks also goes straight up, making the impact of the attacks a sphere rather than a circle, because with grenades, you can hit things through the floor from below. I discovered that in the uncategorized fractal when I tried hitting harpies by chucking grenades from lower platforms straight up. It worked.
It varies.
Some creatures are always in the air, even though their animation shows them on the ground…
Some creatures are always on the ground even though their animation shows them in the air…
The owl griffon, for example….. is the exact opposite of what you described (even though its name appears on the ground, the model’kittenbox is always up in the air)