Windows 10 Setup creates a recovery partition at the disk’s „right end“, as you can see in Windows hard disk management.
You might not even notice that this partition exists, until you want to extend your main partition’s size, e.g. after you have cloned your harddrive to a larger one. While the recovery partition exists, you cannot use Windows HDD management to enlarge a partition besides the recovery, and you also cannot delete the recovery partition within the graphical interface.
Use command-line tool diskpart:
sel dis 0 #your disk number
sel par 0 #your partition number
del par override