Hi there,
Segment and trait population both show the number of devices that qualify for it.
However, segment's population depends upon the Profile Merge rule selected.
If you selected a Profile merge rule as no authentication + current device, then it acts similar to a trait, and will show equal number of devices a trait has.
If you selected a profile merge rule that is current authenticated or last authenticated with any Device Graph option, then segment will show only those devices that will have a real time activity on your web properties.
Please check this doc
Important Considerations for Profile Merge Rules with Device Graphs
Thanks,
Varun Kalra