A patch could be missing from latest Patch Me Security IntelliRollup because it has been superseded by another patch.
Example :
Patch "2020-08 Security Only Update - Windows8.1-KB4571723-x64" is superseded by patch "2020-08 Security Only Update - Windows8.1-KB4578013-x64".
Information could be seen in DSM Webconsole in tab "Superseded By" :
Same information could also be found in Microsoft Update Catalog
https://www.catalog.update.microsoft.com/home.aspx
And patch "2020-08 Security Only Update - Windows8.1-KB4578013-x64" is included in Latest Patch Me security Rollup :
Conclusion :
Generally "Patch Me Security IntelliRollup" contains all Security patches. If an old patch is not present the reason could be that it has been replaced by Microsoft with another patch.