The patches listed in the Patch Management patch catalog are the ones missing on your hosts which were detected using the vulnerability scan. The vulnerability scan provides details about detected vulnerabilities. These detected vulnerabilities can be mapped to missing patches respectively on Linux agent.
After an asset is added to Patch Management, the missing patches details are available in the next vulnerability scan. The default schedule for vulnerability scan is 4 hours.
Alternatively, you can go to the Assets tab to view missing patches on particular assets. Once a deployment job on Linux asset is executed, the missing patches on the assets will be updated when subsequent vulnerability scan is completed.