My matrix isn't showing properly in the pcolor plot

So I am making a program to simulate deposition of a material on a surface. I was testing it and a problem appeared in the display: on the borders of the surface the condition is different from the rest, but the same for them:

if p==1
        a=0;
        b=0;
        for i=1:npontos
            a=a+O(i,p);
            b=b+O(i,p+1);
        end
        if a<=b
            O(a+1,p)=1;
        else
            O(b+1,p+1)=1;
        end
       

this makes it build in a way that if p is allways equal to 1 the result over a period of time (handled by the rest of the program) is two columns formed, on p=1 and on p=1+1=2

I repeated the code for p=100 (other border):

elseif p==100
        a=0;
        b=0;
        for i=1:npontos
            a=a+O(i,p);
            b=b+O(i,p-1);
        end
        if a<=b
            O(a+1,p)=1;
        else
            O(b+1,p-1)=1;
       

the result should be two columns, one in p=99 and the other on p=100, but in this case the column only shows buildind on p=100, despite the fact that the matrix shows that there should be a column in p=99.

Does anyone know what causes this?

Funny thing if I change the border to 99 is works properly.