Why does a student marked "inactive" still count towards my plan?

Because the active students calculation does not look at the Active / Inactive status on the profile: it only looks at activities in the last 90 days (attendance, dues payment, curriculum item learned or validated) and whether the record is recent (less than 90 days old). A student marked "inactive" who came 30 days ago therefore still counts.

For a student to stop counting, they must have no activity in the last 90 days and a record older than 90 days — or be moved to the trash (they stop counting from that day). The "inactive" status is for lists and messages, not for the plan.