Something to keep in mind is that some Intel CPUs fail to hit their max turbo speeds as well, albeit for different reasons. Intel is more specific about what the max turbo is for a given number of cores loaded, and some CPUs will only hit their max speed on a single core. The prevailing theory seems to be that with a typical system there will always be stuff going on in the background, such that there's never truly only one core being loaded, and therefore you may never actually see the single core turbo speed.
I know my 6700k won't hit 4.2 GHz at stock settings running benchmarks like P95 or Cinebench single threaded.