NRR from an ARR table

Table `arr(account_id, month, arr)` has one row per account per month, and churned accounts simply have no row. This query is meant to give NRR from January 2025 to January 2026 but comes out too high. What fixes it?

SELECT SUM(b.arr) / SUM(a.arr) AS nrr
FROM arr a
JOIN arr b
  ON a.account_id = b.account_id
 AND b.month = '2026-01-01'
WHERE a.month = '2025-01-01';