The probably just means the game runs a search for nearby groups before spawning in a new one. If there's one already being simulated nearby, it just moves that one.
Either that, or this is like the land ships I used to see in Combat Flight Sim 3 back in the day. It seemed like all land/sea units were classified as "ground" but only ships would spawn on water, and only vehicles would spawn on land. Didn't stop tanks from driving onto the ocean, or ships from sailing onto land, though. Kinda funny, really.
"Approaching the shore, sir!"
"Jolly good! All ahead flank, Wilkins! And tend to the boiling vessel, if you would, it's about tea time."
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