Different Active Users count when using segments

I would love to understand what I'm looking at - why are the numbers different in this report when I add a segment?

This is the report without any segmentation:

This is the same report with the Mobile Traffic segment:

You are victim of sampling: https://support.google.com/analytics/answer/2637192?hl=en

Sampling applies when:

• you customize the reports
• the number of sessions for the report time range exceeds 500K (GA) or 100M (GA 360)

The consequence is that:

• the report will be based on a subset of the data (the % depends on the total number of sessions)
• therefore your report data won't be as accurate as usual

What you can do to reduce sampling:

• increase sample size in UI (will only decrease sampling to a certain extend, but in most cases won't completely remove sampling)
• reduce time range
• create filtered views so your reports contain the data you need and you don't have to customize them