I would say selecting the correct starting point would be very critical.
Hope the below helps.
My understanding is you select the starting point, the first shot is taken there. The next one [or 2] shots are taken in front of this point and the rest behind it. I recently watched an Oly tutorial and i forgot if they said one or two shots in front of your focus point.
They also mentioned that if your lens reached infinity focus it will stop the bracket or stack.
They also mentioned the differential of 3 was a good starting point. [with olympus lenses] I guess when yours was set at 15 and then 25 the steps may have been too large for the lens you were using, but differential of "4" I would have thought to be ok.
But as
@Bushboy mentioned your lens probably determins the outcome to a certain degree.
and if I understand what you refer to from
@PakkyT that would be correct. you could set a starting point with nothing behind it and it would still step through the incriments till it reached infinity. The lens is not actually focusing on anything, just taking frames at certain programmed spots/steps if that makes sense.
So from my very limited understanding I think they are both correct from a certain point of view [depending if I understand what has been said correctly]
I am still experimenting with this feature with only limited success so far