When I did the test, I wasn't thinking about height, if I recall. But I was thinking about it when I drew it for this post: I became aware some may concentrate on the height, and now you verified it.
There's researchers who suggested something similar to what you said. For example, they suggested females might draw a slanting line to indicate the water level in transition, before it reaches its new equilibrium. Thing is, if you try this live using a bottle or glass of water, you'll see the disturbance is minimal: in other words, tilting the bottle barely makes a difference to the water line, so that even if you drew it the way it was while tilting, still you'd basically draw a horizontal line. Plus, the water line goes back and forth, so it would make sense to draw the average, which would be the horizontal. So if there's some explanation regarding how women may interpret the instructions, it's probably based on a visual illusion that makes us think the water line is less horizontal when it's inside a tilted container.
Or... What if women perceive the world different when tilting their head, than men, so when they see a tilted glass drawing, they might not consider the glass as tilted, but rather the drawing as tilted, which causes them to perceve the gravity in the draiwng as tilted as well.
It would be interesing to see a similar study comparing how women and men read text which is tlted 90 degrees: If they rotate their head, or if they simply read it the way it is presented.
Well that's a bit 'out there' :D