Help to interpret histogram which check datas obtained with particle-tracking code

Hello,

I have to understand a particle tracking code, and at the end of the code, in order to make sure that we have found our features to sub-pixel accuracy, they use this single matlab line given below :

>> hist(mod(cnt(:,1),1),20); %cnt is the centroid of each point I track

This will result in a histogram of the x-positions modulo 1.

When I run my code I obtain this: fig 1 :

or this (fig 2) :

They say : the histogram should look flat if you have enough features and they are not single pixel biased.

I don't really understand what they mean by "single pixel biased" so could you help me to interpret the histograms I obtain? For exemple for fig 2, does that mean I have 120 points with 0.15 as a x-position? and why is my x-axis bet. 0 and 1?

Thank you for your help!! Aude