It has something to do with the rendering. The game only really renders one Black Gum Tree, for instance, and just repeats it. The same for each bush and flower. So the more unique foliage you use, the more unique things it has to render. Use the EA worlds as your guide. For instance, looking at Monte Vista, I'm seeming basically 5 unique trees/bushes used in the world. Not counting individual lots but the EA built lots tend to stick with the 'no more than 5 unique' rule. If you have a great computer setup, you could probably get away with more. But they are setting these up for the basic computer requirements of the game.