Used to be filled every time. They were the quintessential ‘board lesion’. Now, more dentists in general will watch those areas knowing that it is possible for remineralization to occur.
Different approaches. Maybe they remineralized, maybe the dentist saw ‘7’ lesions and figured if you have that many starts you are prone and he wanted to get ahead of the curve before you get a lot more decay starting.
Or, he could be incompetent. That is a possibility as it is in any field. But that you attribute it straight way to that imho shows that you didn’t trust him anyway. I’m glad you have gone to dentists that you trust.

IMO, Too often people attribute a difference in opinion to negligence, incompetence, or worse, intentional malfeasance. Maybe he is. Or maybe he treats it differently. So go to someone you trust.