It also depends on when they determine the start of flowering and when they're ready to harvest. Some people consider the switch to 12/12 as the start of flowering while others consider it when pistils start to show which can take up to two weeks. Some consider a plant harvestable when most trichomes are cloudy while some want a significant amount of amber. There are many variables. Stated flowering times are not reliable and are interpreted differently by different growers or change based on the preference of the grower and when they like to harvest. Environment is also a factor.
The best thing to do is grow your plants until they're the way you like them and then harvest. You can't go by a calendar. Even outdoor growers growing the same strain year after year have different harvest times each year. And some plants of the same strain, clones from the same mother can finish at different times even under the exact same conditions.