What ShineDwarf said is completely irrelevant.
With v1.3c and beyond, normally if the store NPC is placed above water, ground vehicles will spawn exactly on the VehStoreX_landSpawn marker, at the same height as the store NPC. The same applies to heliSpawn and planeSpawn.
If the store NPC is above land, then it will try to find a safe location to spawn ground vehicles within 50m of the marker, with water considered unsafe.