Designer material cost claculation wrong

[Vooker525]

Various shaped hull parts require less materials than cubic shaped ones
Open images to see better

On first screenshot there is tube shaped part


On second one i replaced it with brick shape. After refreshing obstructions design has less total volume but requires more materials to build

[Xantheose]

In the first one, each stone provide 42m^3 and in the second it provide 46m^3.
Weird.