ORGANISM IDENTIFICATION: Helminths

Why can't all helminth eggs be recovered using the flotation concentration rather than the sedimentation concentration?

Some helminth eggs are quite heavy and will not float, even using zinc sulfate with a specific gravity of 1.20. Other helminth eggs are operculated; when the egg is placed in a high specific gravity solution, the operculum "pops" open and the egg fills with fluid and sinks to the bottom of the tube.