Damage to planets

[Q]

As I was testing the unit shields (see corresponding topic) I also tested the planetary shields (massive planetary shield facility in the unmodded game, but I used a facility with phased shields):

The planetary shield works pretty much like shields on ships: Shield skipping weapons like null space projector do skip the shield, shield depleters work as expected and time distortion bombs do 4x damage to the shield. Smart bombs and neutron bombs are blocked by the shield.
Once the shield is down and there are no units in the cargo the population will be killed. In mods that increase the value necessary to kill 1M (standard game = 5) I observed that even weapons that do less damage than this amount will kill 1M. If the value is set to 100 all weapons 1-199kt damage will kill 1M. Residual damage will not be stored for the next shot!! 8 shots with 150kT each will kill 8M and not 12M.
What is still a complete mystery to me is how the facilities are destroyed. Certainly there is a connection with the population loss, but the neutron bomb will kill only population and no facilities. And with standard damage weapons there are sometimes heavy losses of population without facilities destroyed and then suddenly 1M and 6 facilities together are gone with one shot.