An observation on weeds

I put this here because it is a much an observation as a question. I have a lot of plants and therefore a lot of weeds. I have a couple of fairly persistent weeds, but the interesting thing is that certain weeds tend to dominate in particular pots. Two pots side by side both with weeds, but one will have say what we call crab grass and the other will have artillery plant.

I believe this shows how each micro-climate (pot) differs. Though they are basically on the same area, the moisture may be different. Sun exposure (at soil level) & temperature may be different as well. Not to mention what each plant secretes to deter other plants/weeds from sprouting. While in college, I've read an article about the biological warfare being waged by these "innocent" looking plants.

May be the birds or winds or (?) visiting the pots/trees are different, huh?