For example, A good model for the trajectory of a baseball does NOT include the gravitational effects of Venus (but it DOES include the gravitational effects of the Earth)!
Why? Why not include Venus? A couple of reasons:
A given projectile motion model might leave out air friction. The model could be applied to obtain the trajectory of a shot. However, it would be a ``too simple" model if it were applied to the curvature of a ``curve ball" thrown by a baseball pitcher because air friction is the reason the ball curves! This could be shown by playing baseball inside a vacuum! (It wouldn't curve).