Not to get too meta, but what actually makes a well-trained dog decide to come when recalled

I have a newish dog who I've tought basic commands and she will come on command in exchange for a treat most of the time. The other day she was wandering around and I could see her about to follow some scent trail into a part of the yard I wanted her to stay out of, so I told her to come to me. She paused and I could literally see her deciding whether or not to comply–on the one hand, I might give her a treat if she came back, on the other hand, she really wanted to smell whatever was behind the shed.

That got me thinking, what makes a well-trained dog decide to immediately comply with a recall instead of evaluating the pros and cons and deciding whether or not to comply? If I practice 'come' enough times with this dog will she start doing it automatically, even if the 'pros' of returning to me [maybe getting a treat] don't outweigh the 'cons' [not getting to chase a rabbit]?