BigQuery - BI Engine measuring savings

Question

I have deployed BI Engine in one of my Google projects and I am measuring cost savings using the following query

with tbl
as
(
    select creation_time, total_bytes_processed, total_bytes_billed,    
           5 * (total_bytes_processed / 1000000000000) as cost_projected, 
           5 * (total_bytes_billed / 1000000000000) as cost_actual

      from `region-us`.INFORMATION_SCHEMA.JOBS_BY_PROJECT b
     where 1=1
       and job_type = "QUERY"
       and creation_time >= '2022-05-10 11:30:00.000 UTC'
       and creation_time <= '2022-05-10 19:00:00.000 UTC'
)
select sum(cost_projected) - sum(cost_actual) as savings
  from tbl
 where 1=1
;

However, I noticed that very often I have accelerated queries (bi_engine_statistics.bi_engine_mode = 'FULL') for which 'total_bytes_billed = total_bytes_processed'. I was expecting that for accelerated queries total_bytes_billed should be equal to zero which does not seem to be the case.

So the questions are:

1. Is my query the correct way of measuring savings,
2. Is it normal to have fully accelerated queries with total_bytes_billed > 0?