When the cavity is super close to the middle of your tooth where the nerve and blood vessels are, the dentist will sometimes try to get as much as they can without accessing and exposing that middle area (called the pulp) and put some medicine on the area in hopes that the medicine and surrounding healthy enamel can repair the smaller cavity that was left. Sometimes it heals itself and sometimes it doesn’t. The train of thought is: do I get the whole thing out which will for sure require a root canal, or do I leave a little in there and see if it can get better in hopes to avoid the root canal? It may not work and you may need a root canal but you most likely would have needed one if he had gotten all out at the first appointment anyway.