No z-offset that I’m aware of. I made a new project and didn’t change any of the settings except density when I made the tutorial. Is it a new project or a converted one? I converted a project I was working on in 4.7 and the grass did something really weird.
random planes seemed to form going straight up. Could you try to sculpt a hill and see if the grass placement corresponds with the lump? That would tell if it was just placing it at an arbitrary Z value or if it was an offset of the terrain.