I think you are right about the possible movements causing some blurriness.
I expected the flash to freeze any subject to camera distance changes and movements of the subject. I guess there was enough ambient light so the exposure took the full 1/60 sec as the flash was not the main source of light.
The lens is NIKKOR 35mm 1.8 so there is no VR and I did not use any support, it was hand held.
The next time I will try to shoot in manual mode 1/200sec at f2.8 (hand held) I think it should work.