Yea, there are a bunch of theories floating around about that head, Cory...
One follows the belief that the xenomorph head is filled with a fluid which diminishes over time and eventually leaves nothing but the cranial ridging that we see on the Warriors. I personally like that one.
It's also possible that the Drone's head simply changes with time as it molts, and eventually looks like the Warrior's.
Or, as previously mentioned, if the Drone and Warrior are in fact entirely separate, then head shape would have nothing to do with it.
Hopefully AvP2 will shed some more light on this, since the xenos used will be Warriors.