Double hull. Think of it. You can retain the same inner pressure hull, but you can always make quick and flexible changes on the outer hydrodynamic hull. In fact I think the Yuan's inner pressure hull is derived from the Song's, perhaps a lengthened and evolved version from the Song's, hence they share the same 039 designation. Double hull allows the submarine to be malleable, you can make design changes to the outer hull which changes its appearance.
With a single hull, you only have one single pressurized hull and ballast tanks attached to it, either at the waist like in USN nuclear subs, or at the shoulder ballast tanks, like European designs. This thick, high tensile steel needs to be roll pressed, and it gets more difficult to achieve more complex surface geometries for higher hydrodynamic efficiency. That requires higher tech machines to do this. Not a problem for the West, and likely not a problem for China now. Back in the Soviet Union, it was much simpler to do a non hydrodynamic pressure hull with a simple geometry like a cylinder, and then build a complex geometric form around this cylinder with much thinner and malleable plates that can be pressed with lower tech machinery. This inherited into China and goes on until today.
China surely have the machinery today to do single hull submarines, and CSIC (then) offered small coastal submarines for export with single hull designs. Even the Russians have caught up with this, as the Lada/Amur class SSKs are single hulled.
But why still give up the advantages of double hull? The main advantage of single hull is you have more efficient use of space, but some modern submarines are hybrids, such as the Soryu class. Double hull allows you to quickly incorporate external changes to the outer hull while preserving the proven inner hull. That's what we are seeing here with the 039A -> 039B -> 039C.