Use of by_hour_of_day to match UTC revenue do not work

Our revenue data are provided on UTC timezone. To perform our analysis with our Taboola expenses we use the by_hour_of_day dimension to get expenses from day 7pm to day 6pm (Taboola uses ET timezone), details:

1/ To get expenses by campaign for day "UTC", 2 requests:

/reports/campaign-summary/dimensions/by_hour_of_day?campaign=&start_date=&end_date= (we keep datas from 12am to 6pm)

/reports/campaign-summary/dimensions/by_hour_of_day?campaign=&start_date=&end_date= (we keep data from 7pm to 12am)

2/ To get expenses by site for day "UTC":

2.a/ we get all idsite for day and :

/reports/campaign-summary/dimensions/site_breakdown?campaign=&start_date=&end_date=

2.b/ for each idsite:

/reports/campaign-summary/dimensions/by_hour_of_day?site=<site_name>&campaign=&start_date=&end_date= (we keep datas from 12am to 6pm)

/reports/campaign-summary/dimensions/by_hour_of_day?site=<site_name>&campaign=&start_date=&end_date= (we keep data from 7pm to 12am)

But it doesn't work as expected, the total of expenses for an "UTC day" is far (always less) from our estimations (based on an average for few days).

What is wrong?

Thank you!