But building 6 in 7 years is more like building them at war footing. either china has a plan to invade some country(s) after 2020 or china has a clear evidence that it will be invaded by some nations after 2020.
It could also be because China wants to keep its shipyards profitable; the world shipbuilding industry is in a really bad spot due to lowered demand for dry bulk shipping (check the 10 year chart for the Baltic Dry Index). The simplest way to address lowered demand is by feeding naval contracts through those shipyards.
Of course, lowered demand for commercial shipping won't free up any supply for military-specific naval subsystems (radars, missiles, catapults, naval aircraft), which may create bottlenecks - unless China begins importing them en masse from abroad or engages in a large investment program at home.