Then I tell them to sit in a chair and repeat the performance. Then I tell them to lie on the floor and repeat the commands. To step away ten feet. Dogs probably won't sit in place if you are ten feet away. They will probably come to you and sit.
Dogs are SO situational.
It is the trainer's job to make the dog understand that SIT means something very specific in all situations. Let's go back to the trash problem. Dog knocks over trash, has a ball playing and eating. Goes to sleep, plays with cat. Dog crouches, puts ears down, goes to crate, and in general acts submissive, appeasing and guilty.