Since the ordering of the systems is random, system scores obtained by averaging scores of all sentences translated by the system would be unbiased, assuming a sufficiently large sample of sentences.
as shown in Table 5 we can see that the sentences in the "low" partition and the "high" partition have a difference of 0.18, which is highly significant; 3 moreover, this difference is likely to be sufficiently large to alter the rankings of systems in an evaluation.