The problem is...I don't believe the writers thought about it period. If they had, it would have been fairly simple to show us a quick shot of the dropship landing gear with an egg there. That shot alone would have told us it was the queen, that she had laid it on the flight up to the Sulaco, and that the facehugger that came from it is the one in Alien 3. Mystery pretty much solved.
But since none of that happened....the egg got there via crappy writing.