Number of users changes when applying segments in google analytics

I noticed something odd when using session based segments and looking at the number of users in google analytics.

When only using the "All sessions" segment for the month of august I can see that there have been 48 629 users to my site. When I apply another segment to the view so that I can see both "All sessions" and the customized segment, the number of users for All sessions increases to 51 298. The number of sessions stays the same.

I would have expected the number of users to be constant as well. Has anyone else encountered this or knows why it's happening?

The segment I'm using (besides the All session one) is session based and should include all sessions where a certain subset of the site has been visited. It's definition is: page contains /enterprise-products/.

So based on the google support site about this: https://support.google.com/analytics/answer/2992042?hl=en I draw the conclusion that the correct number of users is 51 298 since by using additional segments I have forced google analytics to calculate data on the fly and not use pre-calcualted aggregated data which causes problems for the user-metric. Did I understand that correctly?

There two types of data gathering and manipulation from google. The data you have seen is from #calculation2 in the one case and #calculation1 in the other.

Calculation 1: Pre-calculated data

This calculation relies only on the number of sessions in the given date range and the time of each session. (This is determined by technology managed on the device, like a web browser, and is often referred to as the client-side time.) Because the result of this calculation can be added to the pre-aggregated data tables, Analytics can reference the table to quickly retrieve and serve this data in a report, including when you change the date range.

Calculation 2: Data calculated on the fly

Calculation 2 is based on the way you assign, collect, and store persistent data about your traffic. There are many solutions you can implement to customize this, but the most common way this data is going to be assigned and stored is through cookies managed via a web browser.

Calculation #2 requires heavy computation over large data sets, so it always references data in the raw session tables and not the pre-aggregate tables. Calculation #2 takes more time than Calculation #1 to process and serve data to your reports because the values are calculated on the fly; Analytics can’t simply look up and deliver data that’s already been processed and stored in the pre-aggregate tables. The calculation happens each time you make a request for it. Note that if certain conditions are met, this may induce sampling, but Google Analytics Premium account users can access unsampled reports.

Calculation #2 is used in custom reports and allows for the calculation of Users over any dimension, like Browser, City, or Source.