VisitorID in BigQuery doesn't match Sessions Google Analytics -

i've looked around web , keep getting same answer: count sessions in bigquery, take count(distinct concat(fullvisitorid, string(visitid))). in cases, that's not getting me close sessions in google analytics. there other way count sessions better? here's i'm trying do:

select hits.customdimensions.value val, count(*) pageviews, exact_count_distinct(concat([fullvisitorid], string([visitid]))) sessions [xxx.ga_sessions_20150619] hits.customdimensions.index = 7 , lower(hits.type) = 'page' group val order pageviews desc limit 1000 

for custom dimension values, gets close ga, others off twice amount. there way better session count in bq?

well, can't speak ga data (of course, check make sure you're not sampling data @ all), if run following query, you'll pull sum of each of session counts per fullvisitorid:

select sum(sessionsperuser)   (select fullvisitorid, count(visitnumber) sessionsperuser     [xxx.ga_sessions_2017yyzz]     group fullvisitorid)