That is creating a non-scalar cell array
That is going through each cell member and taking the mean of each. The fact that you did not get an error on this line tells us that each cell member was a scalar or a vector, not empty and not 2 D or more. The end result will be a vector of length k-j+1 = 9.
You are trying to store that vector of length 9 into a single location, Voc_p(i,1)