Well, there is both random and systemic error in any experiment, and if 5ms is small relative to anything you'd expect (or there is some other reason to discount it) then it might be related to a problem in the benchmarking setup that's too small to be worth resolving. Any test is good to within some level of accuracy and they don't always average out to infinitely good if you rerun them enough times.
The 5ms isn't the key number. It's 5ms extra over a 28ms baseline, that's about 18% difference. If your noise threshold is 18%, then I think you have to accept that the benchmark probably isn't any good for this stated task.
https://github.com/bheisler/criterion.rs is good for tests like that. It will give you much more than a single number and handle things like outliers. This makes identifying noisy tests simpler.
