That's weird. my plants are doing the opposite. I have a gorilla zkittles which is an indica dom and its flowering slower than my ak49 which is sativa dom
there's always room for error when it comes to genetics. My GDPs, Purple passions and Blackberry gums took around 63 days, and the ak 420 and GG Autos ran late 70's early 80s. Im mainly going by breeder estimates when mentioning sativa doms though as I've only grown around 8 autos in total, and only three were sativa dom.
You wont be able to tell much about flowering length based on how quickly they stack at the start as I've had some that were slow starters that finished far quicker than others, and some that lagged behind that ended up maturing quickly. its interesting to watch the variation as Autos tend to have a fair bit. I'm around day 28 atm and this one seems to be moving fast. We will see how it turns out.