GPU is a very consistent device, so the purpose of such big sample sizes and multiple launches with averaging is to reduce all the deviations almost to zero. The error is <1% in this case and showing it on the plot will not really change it. The values, however, change when I update the code and improve it, so this is by no means the final way the benchmark will look like. I will think on how to adress this better in the future, but for now I think the best solution if you doubt the results is to launch VkFFT and see what it outputs for yourself.