Creating content through Public API won't update Sub Items total value
I've tested this when creating users, but it should behave the same way for any content object.

See attached image

Steps:

1. open administration interface
2. open a content object (that is container) Container
3. verify that it have <X> Sub Items
4. through public API create a few objects with that Container as parent
5. open the Container
6. verify that it still have <X> Sub Items
7. verify that the newly created objects show on the Sub Items list