Before revision:  These rates depend on many ``local'' factors like the abundance of free ribosomes and cognate tRNA molecules in the vicinity of the mRNA . Consequently, copies of the same mRNA moleculelocated at different parts of the cell may be translated in different rates.
After revision:  These rates depend on many ``local'' factors like the abundance of free ribosomes and cognate tRNA molecules in the vicinity of the mRNA molecule. All these factors are stochastic and their experimental measurements are also noisy.