BigQuery：BQ和GA之间的访问次数不匹配

Question

我的查询是：

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

如果我匹配GA之间的总访问次数（会话），则我具有：

This query:
459950
GA:
491035

为什么？