Clickhouse: RunningAccumulate() does not work as I expect

Question

Say, we have a table testint.

    SELECT *
FROM testint

┌─f1─┬─f2─┐
│  2 │  3 │
│  2 │  3 │
│  4 │  5 │
│  4 │  5 │
│  6 │  7 │
│  6 │  7 │
└────┴──