Currently, I have two plants, a sativa dominant and an Indica dominant. Both treated exactly the same. Same light. The Indica dominant strain has stretched 8 " in 3 days while the Sativa dominant strain has stretched maybe 2". Maybe. Every strain (and, indeed, the phenos of the stains) are different. If you've only grown a handful of different strains, you may not have had a chance yet to discover the possible variations. If, on the other hand, you have 20 grows behind you, each with multiple different strains each time, then I'd be curious as to why. Otherwise, one day you'll see it.