I was wondering why some Win10 (1607) Clients did get the latest cumulative update during a Task-Sequence even if the CU was integrated into the install.wim...

The answer is documented here:

Do not install a language pack after an update. If you install an update (hotfix, general distribution release [GDR], or service pack [SP]) that contains language-dependent resources before you install a language pack, the language-specific changes that are contained in the update are not applied and you will need to reinstall the update. Always install language packs before installing updates.

ouch.. so, offline integration of Updates does not make sense if you are installing a MUI during a Task-Sequence. And if you are not using offline Integration of updates, make sure that you install the MUI before installing updates !