Taken from Gameplay.constants:
table-Aggressive
BuildShip 18
BuildModuleTactical 2
BuildModuleCivilian 1
BuildModuleResearch 3
UpgradeResearch 3
UpgradePlanetArtifactLevel 1
UpgradePlanetPopulation 1
UpgradePlanetInfastructure 1
MaxBountyBidCount 3
UpgradeStarBase 2
BuildMines 1
table-Defensive
BuildShip 13
BuildModuleTactical 4
BuildModuleCivilian 2
BuildModuleResearch 2
UpgradeResearch 3
UpgradePlanetArtifactLevel 0
UpgradePlanetPopulation 1
UpgradePlanetInfastructure 2
MaxBountyBidCount 3
UpgradeStarBase 3
BuildMines 3
According to this, it should be building more than you reported. Perhaps this was a fluke? I'm assuming that those are all various weights as far as what it should do and it's more or less just an RNG as to what it chooses to do. Regardless, is this a consistent issue you're having or no?