This is very,
very similiar to what I already wrote in my Xenomorph biology page...
The queen cannot control what kinds of eggs she lays when she suddenly feels threatened because it takes a prolongued period of time to lay an egg. It must travel through the abdomen for an unknown period of time, develop, and then be deposited.
When the queen is separated from her abdomen, however, she can choose to immediately lay an unfertilized egg through asexual reproduction. This gives rise to a "super" facehugger, which is always female due to the lack of a Y chromosome. "Super" facehugger eggs are hardier in nature, allowing them to be deposited from the queen's pelvis immediately and without nourishment.
And for the record, Praetorians are not considered canon, nor are they considered a true breed in the xenomorph race.
And thus I clothe my naked villainy
With old odd ends stolen forth from holy writ
And seem a saint when most I play the devil.