ORDER BY with no explicit frame defaults to RANGE ... CURRENT ROW, which includes tied peers — running totals jump on tied keys; explicit ROWS is the bar for running sums.

An ORDER BY window with no explicit frame defaults to RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW, and RANGE includes every peer that ties on the ORDER BY key. So tied rows all see each other, and a running total leaps by the whole tie group at once.

SELECT
  t,
  SUM(pts) OVER (
    ORDER BY
      t) AS running
FROM
  scores
ORDER BY
  t,
  pts;

SELECT
  t,
  SUM(pts) OVER (
    ORDER BY
      t,
      pts ROWS UNBOUNDED PRECEDING
  ) AS running
FROM
  scores
ORDER BY
  t,
  pts;

The running total jumps straight to 30 on the first tied row — the default RANGE frame swallowed the peer sharing its timestamp; ROWS adds them one at a time.

State ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW when you want a row-by-row running total, or add a tiebreaker to the ORDER BY so no rows are peers.