Two reasons (or three?) maybe in my eyes why your result is different. #1 set your material setting to "object parametric" not to "world standard" #2 switch "decay near foreign objects" but those two maybe are not so relevant. and now... #3 my guess is that the patch of grass, relative to the object you want to be populated is way too big! Indication? Your only 358 instances. And what you maybe should understand is, that for the population only the center of the object is taken into account. So that you have a lot of overlapping from the patch of grass (sort of border) that covers your gap. You should try a test with for example the carex - and scale it down so that you get something around 10,000 items. Mine btw is the lawn material from Monsoons Ecowear material collection. He created some single grass plants so that this overlapping could not happen. You can see this "overhanging" as well by populating a terrain. Then you see that the patch of grass sort of hangs in the air, if you don't change that in the scale & orientation setting. But you see the "overhanging" too at the borders of your ground object - make a density of something around 20% as a test for that.

One day your ship comes in - but you're at the airport.