Maybe that one's shell was just a little thicker? Who knows... what I can tell you from my experiences is that the *only* time I see the swollen head, swollen neck, swollen belly, and the thick membrane is in my incubator eggs, I never see this in eggs under hens.I kept all eggs in one corner of my bator. I wonder if the bottom most in the corner spot is where this egg sat.
Crap...I should have labeled them. Lesson learned.
One egg left.