Yeah, that is weird. My guess (and that's all it is) is that slugs might behave differently in water than other projectiles.
Daddy loves you. Now go away.
That was what I was arriving at and curious about. My take on it (a guess as well):
Regular JHP's expand due to force within the HP cavity.
Slugs expand because of resistance across the face.
I will note that the "hollowpoint" in every truly HP slug I have ever fired is "blown out" (hydraulic pressure pushes a hole through the front of the slug at this location).
The rest of the slug compacts, and expands radially.
I think maybe slugs do not expand as well in water because they do not meet as much frontal resistance as in gel or tissue.
Mulling it over, that's the best I can come up with. A correlation would be an FMJ pistol round into water vs. gel. You're more likely to see a slight bit of deformation from the gel strike.
From http://www.le.vistaoutdoor.com/downl..._Data_Book.pdf:
Shotgun: 870 Cylinder Bore • Barrel: 18.5 inches • Range: Muzzle 10 ft. from front of gel block
Yes, there are even equations for that 'conversion' if you wish to pursue it.
Both MacPherson (Bullet Penetration) and Schwartz (Quantitative Ammunition Selection) have them; both researchers rely upon modified Poncelet penetration equations correlated against actual gelatin test data.