The number shown is how many posts or comments your instance is aware of. If your number is smaller than the real one, it means that there's content that doesn't exist on your instance.

The missing content is either:

• Old: The account is like 3 or 4 years older tham your instance. Old content doesn't get federated unless someone deliberately asks for it.
• Posted in communities that no one at your instance has subscribed to them yet. This stuff also doesn't get auto-federated.

Maybe they have posts to communities you have blocked?