Leitstelle prioritises a unit with less personnel over the full ones, even if it is 5 meters closer.
As an example: If I alarm a LF/HLF on the west side of station 3, the FL BST 03-HLF20-03 will respond, even though it only has 4 people in it, while the HLF20-01 with its 6 people stays at the station.
If you alarm a LF at the east side of the map, the HLF's respond in next order: HLF-01, HLF-02 & HLF-03, as it should.
This is probably because the game searches the closes fire truck in meters, and the 4 meters that the garages are apart still counts.
Is it possible that leitstelle calculates the nearby firetruck, like it does in a single player game without leitstelle? Without Leitstelle, it looks like the game finds the closes firestation, and alarms it's first firetruck.