BigQuery: различие количества посещений между BQ и GA

Question

Мой запрос:

select SourceMedium,campaign, sum(sessions) as total_sessions from (
    select
    concat(trafficSource.source,trafficSource.medium) as SourceMedium, trafficSource.campaign as campaign,
    fullvisitorid,
    count(distinct visitid) as sessions
    FROM `api-open-broker.108613784.ga_sessions_*`, UNNEST(hits) as hits WHERE _TABLE_SUFFIX BETWEEN '20190101' AND '20190122'
    and totals.visits = 1
    group by fullvisitorid,trafficSource.source,trafficSource.medium,trafficSource.campaign )
group by SourceMedium,campaign

Если я сопоставляю общее количество посещений (сеансов) между ГА, у меня есть:

This query:
459950
GA:
491035

Почему?