If I understand correctly the question is why are there differences being returned in subsequent tests on the Tableau Server. It seems that there are unexpected results for the test series between 3 vizql processes and 6 vizql processes.
A quick look at the results. Test Run ID : 11 looks like it might not have completed properly as the test only ran for 56 seconds and no the full 60. This might skewing some of the numbers that are being seen.
Hope this helps!