if you have ever seen a vintage Breguet Type 20 with a 15 min register work, you know that the chronograph minute hand moves "in two steps": since the original movement with a 30 min register has been modified to have a 15min register, the chronograph minute hand makes "a first step" when the chronograph second hand reaches 55 (then it is between two markers), and a "second step" when the chronograph second hand reaches 5 (then it reaches the marker).
It is a little hard to explain (I should make a video !), but there are a few seconds where the chronograph minute hand is right between two markers. ...and that's axactly what you see on the picture.
So I think the watch has a correct 15 min register.
I hope this helps.