This can happen if you install a component on the same instance, after the instance has been patched. You have an unpatched RTM component (for example: full text search) and SP3 database engine.
If you were running SP3, and recently added a new component (Replication? Full text search? SSRS?) then only those components need patching. You can open the SP3 installer GUI, and step through it to see exactly what component(s) need upgrading. You can then cancel out on the installer without installing.
EDIT: based on the screenshot you uploaded, the "shared features" have been patched to SP3, but the service pack was not completely installed for the Database Engine & SSRS.
You can see the logs for every run of the installer, including incomplete installs. The detailed logs and summary output are located at
%programfiles%\Microsoft SQL Server\nnn\Setup Bootstrap\Log\<YYYYMMDD_HHMM>\. These can be a lot of data to go through, but if you can identify the SP3 install logs from the previous install, that might help explain why the SP was incompletely installed.