Actually, I've seen 2 different theories on this. One theory states that the "Drone" in Alien is more mature than the "Warriors" in Aliens, and that when the Xenos grow, they grow a "cast" over their rib-like cranium, resulting in the Drone. Another theory states that the Warriors are more mature than the Drone, that when the Xenos grow, they lost that smooth cast and reveal their rib-like cranium underneath. I'm more inclined to believe the second one, since the Xeno in Alien has only lived for 24 hours or so, while the Xenos in Aliens has been there since who-knows-when.
We Endanger Species...
Preferably Yours