looks can be deceiving. it's likely just Bioluminescent spots on the hardened upper shell. notice how there is no visible damage there, despite having taken missiles to the back. the more certain kill area would most likely be inside the soft fleshy mouth, and the underbelly, as show cased by the IED attack on it's mouth, which stunned it for a few critical minutes, and the missile to the belly, causing it to vomit up the previously eaten civilians
While APFSDS is more accurate and has more penetrating power, APDS is much easier to manufacture and are more in supply. While overkill is fun, more often than not the military/government strictly adhere to cost and practicality, and would probably only use it if the APDS didn’t work