1/4 or 1/8th moa.
How accurately are you measuring the zenith? Could it be there is a shift, but it's smaller at closer ranges?
Perhaps there's a change in the gun's reaction on the shot.
It's not uncommon to have these sort of steps... i've had them myself before, but try shooting paper from 25-55 and plotting them to the click...sometimes the steps can seem larger than they are or on paper they can be seen as a bad range... although 10 clicks is a hell of a jump... thats the sort of thing I used to see in cold temps, and i used to have to take it all the way back through the zenith... that said, if the shift was 4-5 clicks then i'd only see them perhaps after the zenith...
funny coincidence that the steps i've heard about have always been around 40 yds... never found an explanation for it, and a remark and rezero of the ranges normally makes it dissapear...
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep