There could be scenarios like these:
- an employee changed his or her name
- an employee changed their mobile phone number
- etc …
The profile information in SharePoint is outdated and needs to be refreshed from AD, where the information is more recent.
The powershell command to do this is:
Get-SPUser –Web http://site | Set-SPUser –SyncFromAD
This command cycles through all the SharePoint users and updates the profile. If you come across any errors it is because there are some accounts in SharePoint or on the local server environment that do not exist in SharePoint. If you go into the appropriate SharePoint site you will notice that profiles have been updated.