Using shadowcatcher is far from reality anyway But what's even more off from reality is the plane imitating the grass of the original image. If you would be able to model the grass exactly like it is in the original image, everything would be ok because the self-shadow of the grass should already be in the image and you would only get additional shadow on the blades where the pot casts some. But of course you cannot. BTW: the alpha of the teapot itself is also "wrong" for the same reason.
With shadow catcher you get the shadow cast by scene objects, that's the intention of shadow catcher: to catch shadows. It is no wrong behaviour what you see. If the plane would cast shadows on itself, you would get double shadows - that would be wrong (even though it may be a valid, quick aproximation in this case, that's what you are after).
Additionally you could also say that the result you would like to see (continuous shadow) is wrong because when you compose that part, you would remove valid existing reflections present in the backplate grass as well. So talking about real world in this context is more talking about fakes and workflows.
I'd also like to see some more advanced matte/shadow material with a few more fake-tweaks (include/exclude objects/self-shadow, more shadow control generally) since I regulary hit the limits doing more complex scenes but was not able to turn it into a reasonable and realizable feature request yet.
In your case - depending on the actual scene and detail level - I would propably just remove the noise from the plane and mask the edges with some grass brush in post (or geometry in the scene), you need to do that for the object itself anyway.
Good Luck
