The breeders stated flowering time should only be taken as a guideline. Environment and maturity of the plants at the 12/12 switch have marked effects on the time it takes to finish.
Time to harvest should be determined by the state of the plants (and personal taste), not a stopwatch.
If my plants are ready in the same time as stated by the breeder, then I know I've done something right... I usually have to go an extra week or so.