Hi Ton
It is most likely that the 'Blaar op voet' item has its's own menu item linking to it and therefore modules can be assigned/unassigned to it (in this case unassigned). The 'Suprasorb® P sensitive' item does not have its own menu item and therefore inherits the module assignment from the source menu item (
https://www.startwondverzorging.nl/CONTENT/index.php/26-nieuwe-producten/79-suprasorb-p-sensitive).
If you do a search for 'joomla hidden menu items' you will find details on creating menu items to allw you to assigned/unassigned modules.
You will find the override in the html folder of your template (../templates/[YourTemplate/html)
Hope this helps.
Ciaran