When you make changes to tasks in the app, each Smart List needs to be re-checked to see if the task has to be added or removed from that Smart List. For example, if your Smart List shows high priority tasks, if you change a task from high priority to medium priority, the Smart List has to re-check and figure out that the task doesn't belong in the list anymore.

While a Smart List is re-checking (which happens in the background as you use the app), it won't be able to show an accurate count of tasks in the list. It will show 'smart' until it figures out an accurate task count.