Total Sessions in BigQuery vs Google Analytics Reports

Question

I'm just learning BigQuery so this might be a dumb question, but we want to get some statistics there and one of those is the total sessions in a given day.

To do so, I've queried in BQ:

select sum(sessions) as total_sessions from (
  select
    fullvisitorid,
    count(distinct visitid) as sessions,
    from (table_query([40663402], 'timestamp(right(table_id,8)) between timestamp("20150519") and timestamp("20150519")'))
    group each by fullvisitorid
)

(I'm using the table_query because later on we might increase the range of days)

This results in 1,075,137.

But in our Google Analytics Reports, in the "Audience Overview" section, the same day results:

This report is based on 1,026,641 sessions (100% of sessions).

There's always this difference of roughly ~5% despite of the day. So I'm wondering, even though the query is quite simple, is there any mistake we've made?

Is this difference expected to happen? I read through BigQuery's documentation but couldn't find anything on this issue.

Thanks in advance,

Answer 1

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