Harvest time is a estimate, based on breeder experience in THEIR setups/conditions, across many grows of their strains.
That doesnt mean you, in your grow, will experience same ripening times.
Autoflower has nothing to do with harvest date.. they simply go from vegetative to flowering without the need for photo period change as a stimulus. Once your actually in flowering though, they act much the same way, in that the specific genetics of that seed, and the environment its grown in, dictate when it's at its peak and ready to be harvested