The term flowering period refers to the number of weeks a plant will flower in total, from the moment it comes into bloom until harvesting time. So the number of weeks specified is the average time it takes from inducing flowering to harvesting, NOT to the time it takes from seed to harvest (because that is highly individual, depending on how long you choose to veg).
Bear in mind that the flowering period we specify for each strain is an average. The flowering period can fluctuate because of several factors (whether you're growing outdoors or not, or whether the plants are stressed). Avoid light leaks and nutrient issues to make the flowers ripen more quickly.