Recently, we faced a weird issue in our SharePoint 2013 on-premise farm.
Our SP2013 farm had December 2014 CU applied. We jumped to install April 2015 CU. The CU install went well, as well the product configuration wizard. To verify the CU installation, we ran below PowerShell cmdlet.
(Get-SPFarm).BuildVersion
Strangely, it returned 15.0.4675.1000. It means - December 2014 CU. Not the expected one.
We thought to install more recent CU - August 2015 CU. Again the CU install and product configuration wizard outcome was successful, but the build version remained the same - 15.0.4675.1000. Now that's nasty!
Then, we ran another PowerShell to check the server upgrade status.
Get-SPServer | Where{ $_.NeedsUpgrade -eq $true}
Well, This listed Database server for require upgrade.
Upgrade-SPFarm was the helpful cmdlet then to upgrade the servers in farm. After running Upgrade-SPFarm, it returned the correct build version for SharePoint.
Hope this helps.