Why does the silhouette of a rendered SubD'd prop not match the SubD'd mesh in the viewport?

I'm aware that SubD works best with organic-type shapes, but I love playing. I've just tried using it on a sword prop and noticed an odd effect that I'm hoping somebody can explain.

The picture shows two screenshots and one render of three swords...

Top Sword: I have a simple sword prop PP2 (geometry created in Blender)
Middle Sword: This is the simple sword prop but with SubD applied in DS (Edit > Convert To SubD) - the mesh is almost identical to the bottom sword.
Bottom Sword: I also created a slightly higher resolution version in Blender by applying with one level of multires (Catmull-Clark) and made another PP2.

Left Image: Wireframe view - the middle and bottom sword look the same (as expected) and different from the top sword.
Centre images: Textured Wireframe - the middle and bottom sword look the same (as expected) and different from the top sword.
Right image: DS render - the top and bottom swords render as expected, but the middle sword... ?

(Note: The blade and hilt are separate objects, and are thus SubD'd separately (I guess) - I assume this is why the blade no longer fits the width of the hilt after SubD (in either Blender or DAZ|Studio))

Here's the question: What's puzzling me is this - why does the blade of the middle sword render so differently from the SubD'd mesh it uses?

The middle sword (i.e. the one with SubD applied in DS) is perfect for my purposes (both pointy-end and hilt-end). I just don't understand why... ?

I suspect its the "sharp corner" and "sharp edge" routines. They were not as robust in DS3 as they are now in DS4.

Plus, the preview in DS has always been a very vague approximation of the render, in this case it seems the preview is more accurate then the render.

DS preview being a vague approximation of the render? I'd noticed that -but only as far as materials, lighting, etc were concerned. I hadn't expected it with the mesh itself.

Preview more accurate than render? Yes, it looks that way! But since the render's the final output, that's the most important bit. If I rely on the 'bug' (as I mentioned, the render of the middle sword is perfect for my requirements) what happens if it's fixed? But then I guess DAZ won't be updating DS3 any more?

I guess I should try it in DS4 and see if I get the same effect. But I can't find the SubD - is it a DS4Pro only function?

(On a side note: I know Poser 6 doesn't have SubD - does Poser 8?)

Can't seem to get my prop to SubD in DS4. I'm using DS4Pro, and SubD is there for Genesis (Parameters tab, select Genesis, click All and it shows me Fit To, Resolution Level, Subdivision level, etc. But if I select my sword on the parameters tab and click All it only shows me Point At, Resolution Level, Visible In Render, etc, i.e. no subd.

I guess it's back to the drawing board - with SubD not available in Poser, or in DS4 I think I'll need to up the resolution of the basic model. Blender MultiRes it is then

Shame, the 'unrobustness' of the DS3 SubD rendering made the sword look really nice!