In my experience the answer to your question is really as simple as “it happens”. I wish I could pretend I knew why. I’ve grown the same strain in the tent using the same soil, same feedings and the same training techniques and often find fairly radical differences, like one is slower to switch to flower than another. Personally I feel autoflowering, while very damn good these days, is not perfect. Meaning while they might produce similar looking, smelling and tasting buds, they won’t all grow the same. Some take a longer amount of time to mature than others (like people). But eventually they all grow to maturity. On a side note, I’ve also found the autos that take longer to switch to flower usually grow the largest and yield the most at the end. I’ll add one more note, don’t get over excited about “harvest in 8 weeks” or whatever the breeder claims, I find that is BS in reality most of the time. The plant will tell you when she’s ready to harvest if you know what to look for. Personally I find it common for autos to go 10-12 weeks (or more).