Steve activates the "New Albion/Starpoint" system.
He spends 4 Trade Goods and builds a Space Dock there.
It is legal to build more than 1 Space Dock in a System because they are built orbiting a planet. This system has 2 planets and can sustain 2 Space Docks.
By building his last Space Dock at a location other than "Mecatol Rex", Steve is making a decision about his Secret Objective.
With Nicholas having 2 War Suns, and other ships already there, it seems unlikely that Steve will succeed in taking the planet back. To accomplish his Secret Objective he needs to both control "Mecatol Rex", have a sizeable fleet there and a Space Dock.
By building his last Space Dock elsewhere Steve recognizes he's not likely to accomplish his Secret Objective.