The documentation lacks some details here.
What VirtualBox does every 10 seconds is just slight adjustement (something like 0.005 seconds). Only when the time difference reaches a threshold (20 minutes by default) a “real” resync is done.
You can reduce the thresold (i.e. to 10 seconds) with the following command:
VBoxManage guestproperty set <vm-name> "/VirtualBox/GuestAdd/VBoxService/--timesync-set-threshold" 10000