A possibility: Windows versions may have needed a servicing stack update , like this one https://support.microsoft.com/en-us/topic/kb5003711-servicing-stack-update-for-windows-10-version-1809-june-8-2021-cc836592-f55c-4da9-820b-e4d3c5c5c1d1
And if they don't have it, they are unable to correctly scan for what updates they deserve, when using WSUS / CM.
When you use Windows Update directly, the Servicing Stack may be installed automatically.
Again, this is just a possibility, you asked for "a reason", and that is one of the possible reasons.