Bigquery estimated cost calculated using total_slots_ms is very different than bigquery reservation API billing cost

Question

I'm using this query to find total cost of using slots.total_cost calculated here is drastically different than what I see in BigQuery reservation API. I have standard edition. Am I missing something?

I also estimated storage cost and that does not cover the difference at all. Any help here would be appreciated.

        SELECT (SUM(TOTAL_SLOT_MS)*0.04)/(1000*60*60) AS TOTAL_COST
        , MAX(jobstage_max_slots) AS MAX_SLOTS
        , AVG(job_avg_slots) AS AVG_SLOTS
        FROM
        (
        SELECT
        project_id,
        job_id,
        reservation_id,
        EXTRACT(DATE FROM creation_time) AS creation_date,
        TIMESTAMP_DIFF(end_time, start_time, SECOND) AS job_duration_seconds,
        job_type,
        user_email,
        total_bytes_billed,

        -- Average slot utilization per job  

        SAFE_DIVIDE(job.total_slot_ms,(TIMESTAMP_DIFF(job.end_time, job.start_time, MILLISECOND))) AS  job_avg_slots,
        query,

        -- Determine the max number of slots used at ANY stage in the query.
        -- The average slots might be 55. But a single stage might spike to 2000 slots.
        -- This is important to know when estimating number of slots to purchase.
        job.total_slot_ms,

        MAX(SAFE_DIVIDE(unnest_job_stages.slot_ms,unnest_job_stages.end_ms - unnest_job_stages.start_ms)) AS jobstage_max_slots,

        -- Check if there's a job that requests more units of works (slots). If so you need more slots.
        -- estimated_runnable_units = Units of work that can be scheduled immediately.
        -- Providing additional slots for these units of work accelerates the query,
        -- if no other query in the reservation needs additional slots.

        MAX(unnest_timeline.estimated_runnable_units) AS estimated_runnable_units
        FROM `region-us`.INFORMATION_SCHEMA.JOBS AS job
        CROSS JOIN UNNEST(job_stages) as unnest_job_stages
        CROSS JOIN UNNEST(timeline) AS unnest_timeline
        WHERE project_id = 'open-bridge-bg'
        -- and job_type = 'QUERY'
        --  AND statement_type != 'SCRIPT'
        AND DATE(creation_time) BETWEEN DATE_SUB(CURRENT_DATE(), INTERVAL 30 DAY) AND CURRENT_DATE()
        GROUP BY 1,2,3,4,5,6,7,8,9,10,11
        ORDER BY job_id
        );

https://cloud.google.com/bigquery/docs/information-schema-jobs#calculate_average_slot_utilization

I was expecting total cost to be in the same ballpark as what I see in billing