[1.03b12] Hidden Armour Roll or Bug?

[GodwinW]

My Main Transport is better in every way compared to the Hauler, but has less hitpoints.



I looked through the entire design log and didn't find any location where the default 70 armour was modified by an armor roll, and there's no explicit armor roll in the overview, but something seems to have rolled badly.

Bug or intended?
If intended, I would like to be able to understand how this happened ingame.