Discrepency in Google Analytics between user count in audience overview and users per browser

Question

Having a bit of an issue with Google Analytics. We use it in my workplace for 90+ sites. In the past number of months there is an issue with the count of users.

In the Audience Overview section for a time range, the 'Users' total is 250k. Based on the the previous time for the same period for the previous year it is down 750k. Now, fair enough, there have been changes in the how GA calculates users, not an issue.

However when I create a custom report of Metric 'Users' and Dimension 'Browser' I get a the same total as the Audience Overview 'Users' total but the 'Users' per browser are way above the Audience Overview total.

For example our number browser #1 has 500k users and browser #2 is 275k and so on. When I run the same report for the previous year the totals of each browser match the total 'Users' in the report which also match the total for the Audience Overview report.

Is there a specific reason by the totals are not summing?

Answer 1

This is due to the difference between pre-calculated reports and on the fly reports. Due to the computational requirements to go through the raw data and produce the report the pre-calculated reports are accurate, while the on the fly reports are sampled only to speed up the response of the report. It also depends on the client side time, when that is incorrect the calculations can be off, though I doubt that would be causing such a massive difference in values.