That's because "Fast" strains are a marketing gimmick predicated on a myth that autoflowers somehow flower faster, which they do not. They usually have some ruderalis bred into them under the hypothesis that they will be 'faster'.
So, it's just people putting their thumb on a scale to get the conclusions they prefer due to personal bias. It preys on our 'wants' and doesn't care about following through on what is promised. Not unlike fertilizer promising 'bigger buds' and charging 5-10x more than the ingredients cost in any other equivalent product.
A photoperiod can be as fast as 7 weeks without any ruderalis tainting its genetics. Though this usually correlates with smaller plants, too. Strains that advertise this sort of thing often have a low probability of actually finishing in 7 weeks, but some rare phenotypes will.
The problem pertly lies in the flawed perception of when autoflowers go into flower phase. It happens 7-10 days before you can even recognize any visible clue, so people think an autoflower is faster but it is not. Some think a photoperiod takes a long time to go into flower phase, but that too is not true. If it's more than 1 or 2 long nights, i'd be surprised. The biology inside the plant is what matters, not what the human eye can see, which is incredibly lacking and insufficient. Just a few minutes of light can cause the hormone feedback loop to fail and cause a re-vege... that shows that it's right on the edge and only a minor disturbance throws it off. Either enough of the feedback hormone is produced or it is not... there's not much lag time involved.
Anything an autoflower can do, a photoperiod can do better (one exception). ruderalis genes are more of a weakness than a benefit. They aren't "bad" plants but they aren't special unless you need to worry about light pollution during flower phase -- that is their 1 potential benefit.
again, "Fast" doesn't mean anything too negative. it's got some grandparent or soemthing that was an autoflower buried in its DNA. While i wasn't happy with the potency, the FAST green poison i grew was one of the most robust, fast-growing vege plants i've ever had. It still took ~60 days to finish. I could have measured from the taint and harvested it at 7 weeks, but it wasn't finished. Think i had one finish by 8 weeks from flip. Been too long.
when you have a choice, i'd prefer the non-fast version, but if i had a real hankering to grow a particular strain, if limited to a "fast" version (for whatever reason) wouldn't stop me, either ... to put it in perspective. Just don't believe the marketing people. If it finishes fast, cool... if it takes ~60 days give or takes, i wouldn't be surprised.
