Why does a student marked "active" not count towards my plan?

Because they are not active as far as the calculation goes: in the last 90 days they had no attendance, no dues payment and no curriculum item marked Learned or Validated, and their record is not recent (created in the app more than 90 days ago, imported with an enrollment date more than 90 days old, or imported without a date). The Active / Inactive status on the profile does not count: a student marked "active" with no activity for 6 months is not counted. The displayed enrollment date does not count either.

An Excused attendance does not count as a visit, and neither does a cancelled session. A student in the trash stops counting from the day they were moved there. As soon as they have a new activity (they come back to a class, their dues are paid, a curriculum item is marked), they are active again for 90 days from that date.