First and foremost, don't go by what is advertised as being the harvest time by the seed banks... they want to sell seeds and a quick harvest date helps them do that - but those times are generally reserved for the most expert growers using the most efficient equipment ... i.e., for the Super Growers - LOL! All you need to do is read your own plant and it will tell you when it's time to harvest.
And yes, I think it's probably pretty normal for different plants of the same strain to harvest at different times... a lot of it depends on the light they get and they don't all sit at the same spot under the lights you have... They're individuals, just like children from the same set of parents... they'll grow a little bit differently and respond to the same upbringing a little differently - and you can't say one is right and another wrong....
Your ladies will be fine. Watch the trichs, they'll tell you when they're ready.... Good luck!